Yes, this agrees with my anecdotal observations as well - that it seems more likely to happen when piling up multiple powers and gunfire all at once around the time stasis breaks. It also seems correlated with the "normal" damage bonus not applying, as exemplified in the two vids above.
Yeah I don't know what is going on with these other glitch. I only used Stasis a bit on 2 different runs before, and I don't really remember this insta-death one happening much or at all. I remember killing late game Scions with something like 3 Carnifex shots, but that was a Level 5 damage buffed Carnifex with AP and Critical (I think it would sometimes do it in 2 depending on if you got lucky with critical).
For Vassir, I might have just assumed anything going on there was buggy, since there are some strange things that can happen if you ragdoll her, and sometimes if you throw her off the ledge you can completely bork the mission.
I wonder if this insta-death glitch is more likely on targets that have some sort of plot triggers associated with them. In my case the two that glitched like this both have triggers associated with them (Jaroth and Jedore). I did use Stasis on Garm in that same mission, although I didn't shoot him until he hit the ground. And in Purgatory I don't think it did anything unusual to the YMIR and the couple of higher tier guards I used it on.