Just now, loading the page https://lemmy.world, I saw a different user’s main page. The page loaded in light mode (I use dark) with the username of /u/froodloop in the top right. Then a moment later, it refreshed into my expected main page with my username in top right. This went past too quickly to get a screenshot, but it was reminiscent of some of the bugs that were happening in the old websocket code.
I’m not sure if the link text has to be correct for the mention. When you select from the dropdown box on lemmy it comes out with @user@instance in the link text.
Also I edited my comment above to include instance agnostic links. Unfortunately they don’t deliver mention notifications.