[Bug]: `:Mattermost:` Emoji Cannot be Used in Reactions

Before you file a bug report

• I have checked the issue tracker and have not found an issue that matches the one I'm filing.
• This issue is not a troubleshooting question. Troubleshooting questions go here: https://forum.mattermost.com/c/trouble-shoot/16.
• This issue is not a feature request. You can request features and make product suggestions here: https://mattermost.uservoice.com/forums/306457-general/.
• This issue reproduces on one of the currently supported server versions.
• I have read the contribution guidelines and the Mattermost Handbook Contribution Guidelines.

Mattermost Server Version

10.11.9 and 11.4.0

Operating System

Fedora 43 (Desktop [Flatpak v6.0.2] and Web [Firefox v146.0])

Steps to reproduce

1. Go to a message
2. Click add reaction
3. Select the Mattermost emoji (:Mattermost:)

Expected behavior

The :mattermost: emoji should be added as a reaction to the message.

Observed behavior

No reaction is added.

Log Output

Attached

Additional Information

I've tested this on our team's internal server and the Mattermost community server. It is not fixed by clearing the cache. I have had our admin verify that they can reproduce this on their configuration as well. They were able to verify the bug exists in their web client for our server.

The issue appears to be one of capitalization. Both of my clients on desktop (Flatpak and Firefox) attempt to react with :Mattermost:, however, using the client on my phone (Android 16 App v2.35.0) reacts with :mattermost:, which works and is visible to all clients.

Using the emoji in a message works with either :Mattermost: or :mattermost: in the desktop client, but only as :mattermost: on mobile.

main.log.txt