@Kafka Sorry, I kind of cross-posted from here 
(but half-successfully):
I just thought there was a chance it might be related.
This was a Level 12 Explore in Suncrest – 4x speed, Android (old-ish).
No particular weirdness that I recall, tbh – this was just after an Egg Thief cast, so I noticed damage being taken from destroyed skulls, but the Life value not changing. Before that, I would have probably cast Skeleton Key, killing the bottom enemy and damaging Harpy Mage. Probably.
I’ll keep an eye out and let you know if I see it again or find a way to reproduce it reliably.
@Earnham do you still have any of those videos? I’d be interested to see if something similar happened prior to the Egg Thief insta-kill from.your original post.