I have never heard of that issue before and looking through the code I'm not seeing any reason that should happen. Can you give some more details? Where were you prior to telling him to stay behind? Where did you go next?
At a minimum, you could try this but not sure if it'll work:
#DebugMode 1
##dm_spawncreature terinus23
##DebugMode 0
May have some plot stuff messed up.
That said, Terinus winds up magically getting teleported to you when he's needed anyway, even if you never take him from the tower. So you may be fine without him if you can hanle the combat.
It was right after he opened up the Hall of Worlds and he goes in there, I follow, and he appears as my henchman as soon as I appear in Hall of Worlds. I removed him from my party since I want to go solo and he does the old "fine, I'll be right here if you need me again" and he's standing at the door to Blackwood Tower in the Hall of Worlds. So I go off to the Etherial Plane to get some goodies from the dragon, come back, and he's not standing in front of the door to Blackwood Tower anymore. He's nowhere to be found, not in the Hall of Worlds, not in Blackwood Tower, nowhere.
It's weird because I know I've done this sort of thing before in previous games, so it's weird to me that he would just disappear where I left him. I even cast true sight to see if he was invisible standing there but no.
Well, I'll try that console command. I guess if he gets put back into the game when he's needed for plot reasons then that's okay because my game won't be broken. It would just be nice to have him so he can enchant some of my weaponry.
Thanks for the suggestions.