Both the DE and Undead variant.
Reproduce:
Start the .exe
research 'Slave Horde'
Save
-> works
Quit to main menu
Switch hero
Start skirmish and resign
switch back and reload save
-> doesnt work
Edit: I was too impatient, it works, after an eternity (did I mention that I'm impatient?). Still, something's fishy here, it works instantly in a fresh game
Edit2: same thing with chaos magic transform