See: forums.garmin.com/.../since-last-forum-maintenance-update---unable-to-tag-mention-other-forum-users
Hi, little complex workaround ... genius
FYI, a user contact me by DM and explain that after 10-15 try sometimes it works ...
But I can tag anyone where. For example Brandon.ConnectIQ Or is that only because I'm admin?
for sure, account with a different level of rights could explain a lot of things
Or is that only because I'm admin?
Well let's consider the alternatives:
1) Anyone can currently tag anyone anywhere, same as it's always been, and we're going to all this to trouble to complain about a problem that doesn't really exist, as well as finding and documenting a workaround, for absolutely no reason
2) You (and other admins) are the only ones who can currently tag anyone anywhere
What's the simplest (or most likely) explanation?
I know your question was probably somewhat rhetorical but still.
Here's some "evidence" - when I type "@" and allow the autocomplete popup to appear, it only shows people who are in this thread. I bet this was done as some kind of optimization, considering that this popup has typically taken a very long time to populate in the past. Unfortunately, it also means that manually typing @some_known_username does not work at all if some_known_username is not in the current thread. Ofc, in the past, tagging someone in this way (by manually typing in their name) still wouldn't work if you didn't wait for the autocomplete popup to appear and match the name you typed. Iow, tagging is just inefficient/broken by design and/or implementation details that won't change, and this apparent workaround to make it more efficient has only made it worse.

Or maybe it really is a deliberate choice by Garmin to prevent non-admin forum users from tagging people who aren't in the thread. Either way, it's kind of annoying.
Maybe you should create an alt/burner account so you can experience the same forum as everyone else, when it comes to looking at issues like this, if only for your own interest. You could even make it clear that it's an alt account of yours - name it "flocsy nonadmin" or something.