I discovered that if you hover over your chars, as shown:
Then enter a portal with your cursor still hanging on any of your chars, still previewing, it’ll stick to your cursor in the realm, and stays there regardless of moving or shooting:
Even if you nexus, it hangs around until you preview your chars again:
Easy enough to fix yourself if it happens without resetting the game, but enough to warrant a notice that the bug exists, I suppose.
The real question is why I bother finding these lol