I just had an odd variant of this:  I lost several +Tamed units in tactical combat fighting Vetrar (only one of which, a pack of Ice Wargs, were +Tamed in that battle -- three other +Tamed beasts, all Black Widows, had been +Tamed in previous tactical engagements).  All of them returned with 0 HP ... however, upon entering another Tactical combat, all the 0-HP units appeared initially, but despite healing one with my Sovereign before it had its first turn, every single one died after completing its first turn (though none had been hit by an enemy).