So! The newest Discord mobile update is absolutely terrible. Now, instead of tapping someone's profile picture to mention them, you need to tap their username. I'm linking a thread on Discord's feedback site about this issue, please respond there and help us get that feature back! If you don't want to make a full response, that's fine! Please at least visit the thread and upvote the current responses!
https://support.discord.com/hc/en-us/com...ntion-them
Attached is an example of the tap hitboxes on someone with a single character for their nickname - as I'm sure you know, it's also possible to have a completely blank name, or at least it used to be.