Monsters unable to perform tactical summons in current version of the game. Maybe computer players too but I haven't seen that in action yet. This is funny because this is something that has worked fine in earlier versions of the game for as long as I have played it. My own heroes can perform tactical summons and it works properly. But if a monster performs a tactical summons, the game doesn't lock or crash or anything, the spell appears to complete correctly but no additional enemy appears. This happens every time, no exceptions.
The bug happens if the tactical AI has control. So all enemies are affected and your own units too if you use autoplay.
Stardock supposedly has an internal fix but they are not releasing it.
There have been quite some posts on this like: https://forums.elementalgame.com/498192/