Reproduction steps
0) Ensure the account does not have the "Use external emoji" permission
1) Attempt to add a new reaction to a message using an internal custom emoji from the community.
Expected behaviour
The emoji appears and remains, since the user has add reactions permission.
Actual behaviour
The reaction emoji appears and disappears in a flash.
Build information
The build I reproduced this on is:
Canary Web 2026.1017.223811, Linux (x64), Firefox 154.0, Locale en-US
Platform
macOS, Windows, Linux, Web, Android, iOS
Evidence
I personally tested on Canary web but this was originally reported to me by a community member and me and another member were both able to reproduce it.