I just had the swamp drake issue but with L sized enemies:
In the map Soultree, when escorting Viridya up the hill, I ran into two stonekin warriors that became immune to damage. I was using swamp drakes as well.
I've attached my log & replay, the issue is very obvious at the end of the replay.
I remember coming across units that I couldn't hurt with swamp drakes before in other maps, but after they moved a little they could be damaged again. This instance stood out to me because it caused me to lose. I don't think it has anything to do with cc but where the enemy is located on the landscape, maybe the swamp drake attack splash somehow fail to connect with the enemy hitbox.
_log_proxy_latest.log Immortal_enemies.pmv