While playing necro I added a skill point into Summon Skeleton Mage’s Pyromancer skill. I was able to summon 3 pyromancers, and 2 standard mages. I respecced the skill point (removed it) in Pyromancer and upon summoning new skeleton mages, they are all now pyromancers.
I will try to repro this in a bit, admittedly I am now having fun with this bug. I will update with details if I can find a pattern after a log in/out.