When you @mention somebody and your mouse is in the area of where the suggested people show up, the UI tries to select what was under the cursor. If your mouse is off to the side when you @mention, it behaves perfectly (to me).
My use case is almost always to use keyboard only until the first suggestion is the best, and then I hit enter. However if my mouse is in the wrong spot (which it often is, because I clicked in the text box to start typing) it will go straight to selecting what’s under my mouse.
Idea for a fix: Wait for a mouse movement before assuming the @mention suggestion under my mouse is the one that I want. Done! Low effort, ultra high impact for sure!