Regular enemy health going grey and not taking damage (not in void fissure/storm missions, not Kuva guardians, not any special enemy types)

[Vesciroth]

So I've been encountering this for a few weeks at least now, and I only play sporadically so I was wondering if this is a new feature I just missed or a bug. Based on feedback from this thread I made in general discussion, it seems like a bug.

Basically, with no indication as to why it's happening as far as I can tell, regular enemies' health bars will turn grey after I do a bunch of damage to them and they just become completely damage immune. It just happened to me again on a Steel Path Incursion (Grineer Defense, not on Kuva Fortress). There was no acolyte around, and tons of mobs just had their health bars turn grey for no reason. I had to go around searching for yellow health bars for me to try to kill, and some of them started turning grey too. Eventually the effect seems to wear off and I can finally kill them, but it seems to last a good 5-10 seconds, maybe longer in some cases. Other people in that thread have had similar experiences.

The enemies do not stop attacking at all when this happens, unlike how they behave in void fissure/storm missions, so it is much more inconvenient as they are able to continue attacking while you're unable to damage them.