Was running NM Cow Level solo. My Oak Sage would become invincible, which was great for taking focus off me and not having to worry about buff falling off if it died. I’m not sure why it would become invincible. I do know that at the start it would take damage as normal and even die. I had a follower as well, Act II Holy Freeze, which took damage as normal and did not have the same invincible bug.
Haven’t had the same bug happen anywhere else that I’ve noticed so I’m not sure that this is unique to Cow Level. I’ll update if I find it happening elsewhere.