VTT Monsters being removed from an encounter mid-combat

I was running a VTT session this morning and there were creatures randomly disappearing from the encounter and initiative order with no way of adding them back. The tokens would stay on the map but the only way to get them back in was to spawn a new creature. This happened multiple times and seemed to be caused by changing their HP but could have been caused by something else. The monsters in question were Allip and Shadow (legacy).

This happens with me, if I open their stat block and close it, but I accidently use the "X" button instead of the back arrow.  Only way I can fix it and get them back into initiative is to end combat, wiping the initiative board and adding all the tokens back to it, but you will have to redo initiative.

I will have to try that. Thank you!