My experience in the Anubis temple has been that the further in you go, the less likely they are to work. I’m wondering if it may be related to specific creatures as they spawn?
I was there twice this weekend and didn’t have any fireballs fail in the first rooms to the east and west of the entrance.
When advancing to the next area after the trapped halls, we gathered all of the disciples from the four rooms around the locked/lever-released hall. When using fireballs on that group, they would effect some of the enemies, but not others. It wasn’t related to a spacing issue, as some of the enemies closest to the detonation were unaffected, while those further away took damage. There were no Acolytes in this group that would have had SR.
In the northern most room, after the locked hallway - the fireballs always seem to work on the three disciples in the main area by the altar. The Anubis Guardians are immune, but the combat log reflects that, so they seem to be getting hit.
In the rooms to the east and west of the altar, fireballs work sporadically again - effecting some targets and not others.
In the hidden room with two chests, I can’t get any AoE damage spell to work at all. Fireballs, firebrand, and ball lightning all produce an animation but don’t do anything else.