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 locate
