Federation: mentions of federated users render as plain text (and as @@user on mobile)

Description:

In a federated (Matrix) channel, mentioning a user from a remote server is effectively broken from a rendering standpoint:

• On the sending side, the mention renders as plain text (e.g. @user:server.org) with no mention pill/highlight, so there is no visual confirmation the mention actually worked.
• The mention is delivered and rendered correctly on the receiving side (the remote user gets notified) — so this appears to be a rendering problem, not a delivery problem.
• Quoting a message that contains mentions does not render the mentions in the quote (plain text again).
• On mobile it goes further: quoted mentions render as @@user (double @), and selecting a federated user from the mention autocomplete inserts @@user:server.org into the composer.

Steps to reproduce:

1. Have a federated channel open between two servers (Matrix federation).
2. From server A, mention a user from server B (e.g. @user:serverB.org) via autocomplete and send.
3. Observe the message on the sending side: the mention renders as plain text.
4. Check the receiving side (server B): the mention renders correctly and notifies the user.
5. Quote a message containing mentions: in the quote, mentions render as plain text.
6. On the mobile app: quote the same message: mentions render as @@user.
7. On the mobile app: type @, pick a federated user from autocomplete: the composer text becomes @@user:server.org.

Expected behavior:

• Mentions of federated users render as proper mention pills/links on both the sending and receiving sides.
• Quoted messages render mentions the same way as regular messages.
• Mobile composer/quote handling produces a single @ mention.

Actual behavior:

Mention of a federated user renders as plain text on the sending side — no indication the mention worked:

The mention does work on the receiving side — it renders and notifies correctly (note: the double mention here was only because we weren't sure mentions were working at all):

Quoting a message with mentions doesn't render them either (plain text inside the quote):

On mobile, quoting renders the mentions as @@aaron:

And on mobile, selecting a federated user from the mention autocomplete inserts @@sing.li:rocket.chat:

Server Setup Information:

• Version of Rocket.Chat Server: 8.6.0
• License Type:
• Number of Users:
• Operating System: Kubernetes (official container images)
• Deployment Method: Official Rocket.Chat Helm chart (latest chart version), microservices deployment
• Number of Running Instances: multiple (microservices)
• DB Replicaset Oplog: enabled
• NodeJS Version: as shipped in the 8.6.0 image
• MongoDB Version: 8.0.3

Client Setup Information

• Desktop App or Browser Version: reproducible in the web/desktop client; mobile behavior observed on the Rocket.Chat iOS app
• Operating System: macOS (desktop), iOS (mobile)

Additional context

• Federation is Matrix-based; the channel is federated between our server and a remote server.
• Happy to share our server/homeserver details privately if needed for troubleshooting.

Relevant logs:

N/A — no related errors observed in server or browser logs while reproducing. Happy to collect specific logs on request.

Start by reproducing the issue in the web/desktop client and iOS app across normal messages, quoted messages, and mention autocomplete. Trace the federated mention rendering and mobile composer/quote handling from those entry points. Done means federated mentions render as mention pills or links in messages and quotes, while mobile insertion and quoting produce a single @.