Have a look at the screenshot - somehow PiB managed to get five minions, which is my understanding that this should not be allowed.
Not sure how to reproduce it exactly, but I do have a feeling that rapid summoning of multiple minion generators is the cause.
By that I mean rapid clicking the hero’s special weapon simultaneously or in quick succession.
Using iPad Pro with latest iOS and game version.
Hope this helps!
Btw - it does look cool having five minions though!