I don't know if it is a known bug yet, but in case it isn't, I wanted to make sure it gets fixed sooner or later. 
Anyway, just a few minutes ago I ran into the enemy sovereign, who had summoned the Stone Giant and his Spy.
I clicked on his stack to see what units he had with him and there was this tab called "enchants", you know, the one you also have on your own channelers... so I thought 'cool, lets look what he casted until now'. That's still okay, but then I was able to select each enchantment and by clicking on it I got the message "Would you like to dispel Rokoth's Promise cast on Relias?" - well, I said yes and... erm, his unit was gone. Same thing with his other summoned unit.
I mean, that gave me the chance to beat the crap out of Relias, but I'm sure that wasn't how it was supposed to be, right? It didn't cost me any mana and I had no dispel magic spells (are those even there?), so I figured it had to be some kind of 'unwanted feature'.