Hello,
I see something like this was reported previously, but I just wanted to reiterate. When selecting the Death Knight passive, and then casting skeleton mages again, you do get a replacement for most, but a solitary Skeleton Mage will not get replaced.
You have to remove the skill to remove the mage.
I also am seeing summons remain out after despecializing a tree too. I had a Blood Golem, and decided to remove the specialization for Golems for Skeleton Mages, but the Blood Golem remains. I have not removed the skill, or recast, so the game is not checking.