Platform (Android, iOS, Website): web
Browser, if a website issue (Firefox, Chrome, etc) : chrome
Step 1: Make a comment on an observation.
Step 2: Edit the existing comment, adding an @ mention. Use the drop-down to choose the username without typing the whole thing in. The bit that you typed will get replaced with the full username, but after saving the edit, it will revert to only the amount that you had manually typed in.