Mention autocomplete which allows the user to see a list of users in the current channel after typing @... does not pop up the users list on mobile devices, it works fine for the same user on desktop app / native web browser.

It does work on mobile only if the user has the view-d-room & view-outside-room permission but then he can see all the users on the server not only the users in the relevant channel.

How to reproduce, verify your user does not have the view-d-room & view-outside-room permission, join a channel and try to mention other users by typing @ + first letter
On desktop app/web it works as expected

Try the same on mobile, does not work