There are 6th floor mobs spawning on the first floor. They see invis and may possibly be messing up the key mob. Also , the name spawn rate seems to have went from 1000 to 0 instead of somewhere in the middle.
Had two groups killing in TOFS for hours last night. Only 1 named (Deacon Foels Tomorin) spawned. Not zero, but huge change for sure. Anyone that dots a mob and evacs can bring higher lvl mobs to entrance. What specific mobs did you observe on Floor 1? Can you rule out an evac that caused mobs to warp over?
On the same vein, had 5 a shadow beast necromancers up in ToFS 6b on Xegony. Looks like 3 of the shrouded bats are actually necromancers (the actual key mob is not in his spot).