I dont know for sure why this happens, but it has happened to me twice already. Both times in late castle age/imperial, once yesterday in 1v1 mp, once today in skirmish/single player.
Upgrades for my scouts: both games i had professional scouts researched, both games castle/imp, with chinese (i havent tested other civs)
Tried deleting other units, and it worked correctly shown here:
But when i try to delete my scouts this happens :
you can clearly see the scout with 0 hp, they do the animation as if they were killed and after the animation they regen their hp back.
They can be killed normally by the enemy, but i cant delete them.