Playing on Xbox One. I started a level 1 explore in Dhrak-Zum with Gnome-a-palooza active. There were a few battles with 3 gnomes and a Doom of Stone. It appears that gnome-a-palooza is not properly replacing this troop (and possibly other dooms) with gnomes.
Next time I’m able to craft a gnome-a-palooza, I’ll test explore in other kingdoms with a doom troop.
I noticed this on Android.
I believe the reward for 1 gnome was also missing.
Same with Doom of Nature and Bright Forest on PC
Hey,
This is a currently a Known Issue, in regards to Gnomes not replacing Doom Troops and we are working on a fix!
I have updated the Known Issues article to reflect this as well.
Jeto - Support Team Human 
Why are doom troops considered heroes?
My apologies, I was meant to write Doom Troops. Responding to a few similar GAP issue.
I’ll edit that now!