Flame Sentinel challenge issue

When doing the Kill Rare monsters in Cruel challenge I saw that I only needed to kill a rare Flame Sentinel to finish it. I went in the Solaris Temple, found one on level 1 and killed it ... challenge still incomplete. I then went in Cruel just to be sure and killed another rare Flame Sentinel, still incomplete.

Note that I am 100% sure these were flame sentinels (checked the wiki and a normal flame sentinel to confirm), and that they were rare (glowing yellow aura, mods ...). Is this a known bug ? I can't find anyone with the same issue.

There are unique flame sentinels in Solaris. You sure you had a rare one and not a unique one?

Yes the rares I killed were not the twin uniques that are on the first floor. I tried to kill them as well just in case that counted but as expected it didn't.

Well I went in again, found another one and got the challenge this time. Not sure what happened the first 2 times, probably my mistake.

Are you sure it wasn't a lightning sentinel? Their base artwork is more or less identical; the only way to tell the difference is what spell they cast. Yes, they are two distinct monsters. I wound up having to run a bunch of Solaris instances before I actuall got a rare FLAME sentinel to spawn; tons of lightning ones, though.