A ship appears to be invisible. (Or doesn’t appear, as it were.) This also creates weird issues with merged fleets.
Ship in transit (normally you see a ship, but here you see nothing - this is the ship traveling in a planetary system otherwise background would be warp background I think)
Fleet view (information edited out appears to be correct)
If the invisible ship is the flagship when other ship(s) are with the fleet, again, that ship is invisible, but other ships appear normally. And attempting to switch flags by switching positions of ships in fleet creates error message. No screenshot of that though.
Workaround - merging the invisible ship with a normal ship so the normal ship is the flagship seems to maybe resolve the issue. Invisible ship reappears. Did for me anyways.