diff --git a/src/@types/common.ts b/src/@types/common.ts
index 991cfb7e08..d70d80fcd5 100644
--- a/src/@types/common.ts
+++ b/src/@types/common.ts
@@ -34,3 +34,10 @@ type Prev = [never, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ...0[]];
 
 export type Leaves<T, D extends number = 5> = [D] extends [never] ? never : T extends object ?
     { [K in keyof T]-?: Join<K, Leaves<T[K], Prev[D]>> }[keyof T] : "";
+
+export type RecursivePartial<T> = {
+    [P in keyof T]?:
+    T[P] extends (infer U)[] ? RecursivePartial<U>[] :
+        T[P] extends object ? RecursivePartial<T[P]> :
+            T[P];
+};
diff --git a/src/components/views/rooms/EventTile.tsx b/src/components/views/rooms/EventTile.tsx
index 28073fed32..92c783bfce 100644
--- a/src/components/views/rooms/EventTile.tsx
+++ b/src/components/views/rooms/EventTile.tsx
@@ -1403,11 +1403,8 @@ export class UnwrappedEventTile extends React.Component<IProps, IState> {
             msgOption = readAvatars;
         }
 
-        const renderTarget = this.context.timelineRenderingType === TimelineRenderingType.Thread
-            ? RelationType.Thread
-            : undefined;
-
-        const replyChain = haveTileForEvent(this.props.mxEvent) && shouldDisplayReply(this.props.mxEvent, renderTarget)
+        const inThread = this.context.timelineRenderingType === TimelineRenderingType.Thread;
+        const replyChain = haveTileForEvent(this.props.mxEvent) && shouldDisplayReply(this.props.mxEvent, inThread)
             ? <ReplyChain
                 parentEv={this.props.mxEvent}
                 onHeightChanged={this.props.onHeightChanged}
diff --git a/src/components/views/rooms/SendMessageComposer.tsx b/src/components/views/rooms/SendMessageComposer.tsx
index d4837c8401..35a956f09d 100644
--- a/src/components/views/rooms/SendMessageComposer.tsx
+++ b/src/components/views/rooms/SendMessageComposer.tsx
@@ -57,12 +57,12 @@ import { ComposerType } from "../../../dispatcher/payloads/ComposerInsertPayload
 import { getSlashCommand, isSlashCommand, runSlashCommand, shouldSendAnyway } from "../../../editor/commands";
 import { KeyBindingAction } from "../../../accessibility/KeyboardShortcuts";
 import { PosthogAnalytics } from "../../../PosthogAnalytics";
-import { getNestedReplyText, getRenderInMixin, makeReplyMixIn } from '../../../utils/Reply';
+import { getNestedReplyText, makeReplyMixIn } from '../../../utils/Reply';
 
 interface IAddReplyOpts {
     permalinkCreator?: RoomPermalinkCreator;
     includeLegacyFallback?: boolean;
-    renderIn?: string[];
+    inThread?: boolean;
 }
 
 function addReplyToMessageContent(
@@ -72,7 +72,7 @@ function addReplyToMessageContent(
         includeLegacyFallback: true,
     },
 ): void {
-    const replyContent = makeReplyMixIn(replyToEvent, opts.renderIn);
+    const replyContent = makeReplyMixIn(replyToEvent, opts.inThread);
     Object.assign(content, replyContent);
 
     if (opts.includeLegacyFallback) {
@@ -131,8 +131,8 @@ export function createMessageContent(
     if (replyToEvent) {
         addReplyToMessageContent(content, replyToEvent, {
             permalinkCreator,
-            includeLegacyFallback: true,
-            renderIn: getRenderInMixin(relation),
+            includeLegacyFallback: includeReplyLegacyFallback,
+            inThread: relation?.rel_type === RelationType.Thread,
         });
     }
 
@@ -398,7 +398,7 @@ export class SendMessageComposer extends React.Component<ISendMessageComposerPro
                         addReplyToMessageContent(content, replyToEvent, {
                             permalinkCreator: this.props.permalinkCreator,
                             includeLegacyFallback: true,
-                            renderIn: getRenderInMixin(this.props.relation),
+                            inThread: this.props.relation?.rel_type === RelationType.Thread,
                         });
                     }
                 } else {
diff --git a/src/utils/Reply.ts b/src/utils/Reply.ts
index 0eccc356df..d8b6abc5ee 100644
--- a/src/utils/Reply.ts
+++ b/src/utils/Reply.ts
@@ -14,13 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { IEventRelation, MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { IContent, MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { RelationType } from "matrix-js-sdk/src/@types/event";
 import sanitizeHtml from "sanitize-html";
 import escapeHtml from "escape-html";
 
 import { PERMITTED_URL_SCHEMES } from "../HtmlUtils";
 import { makeUserPermalink, RoomPermalinkCreator } from "./permalinks/Permalinks";
+import { RecursivePartial } from "../@types/common";
 
 export function getParentEventId(ev: MatrixEvent): string | undefined {
     if (!ev || ev.isRedacted()) return;
@@ -141,21 +142,18 @@ export function getNestedReplyText(
     return { body, html };
 }
 
-export function makeReplyMixIn(ev: MatrixEvent, renderIn?: string[]) {
+export function makeReplyMixIn(ev?: MatrixEvent, inThread = false): RecursivePartial<IContent> {
     if (!ev) return {};
 
-    const mixin: any = {
+    const mixin: RecursivePartial<IContent> = {
         'm.relates_to': {
             'm.in_reply_to': {
                 'event_id': ev.getId(),
+                'io.element.is_falling_back': !inThread, // MSC3440 unstable `is_falling_back` field
             },
         },
     };
 
-    if (renderIn) {
-        mixin['m.relates_to']['m.in_reply_to']['m.render_in'] = renderIn;
-    }
-
     /**
      * If the event replied is part of a thread
      * Add the `m.thread` relation so that clients
@@ -173,19 +171,12 @@ export function makeReplyMixIn(ev: MatrixEvent, renderIn?: string[]) {
     return mixin;
 }
 
-export function shouldDisplayReply(event: MatrixEvent, renderTarget?: string): boolean {
+export function shouldDisplayReply(event: MatrixEvent, inThread = false): boolean {
     const parentExist = Boolean(getParentEventId(event));
+    if (!parentExist) return false;
+    if (!inThread) return true;
 
-    const relations = event.getRelation();
-    const renderIn = relations?.["m.in_reply_to"]?.["m.render_in"] ?? [];
-
-    const shouldRenderInTarget = !renderTarget || (renderIn.includes(renderTarget));
-
-    return parentExist && shouldRenderInTarget;
-}
-
-export function getRenderInMixin(relation?: IEventRelation): string[] | undefined {
-    if (relation?.rel_type === RelationType.Thread) {
-        return [RelationType.Thread];
-    }
+    const inReplyTo = event.getRelation()?.["m.in_reply_to"];
+    const isFallingBack = inReplyTo?.is_falling_back ?? inReplyTo?.["io.element.is_falling_back"];
+    return !isFallingBack;
 }