What you were expecting to happen, and what actually happened
What I was expecting was a x2 bonus to magic. I received no bonuses anywhere. Not on my hero, not on my team (which all are even purple to match Minishroom’s color), there were no bonuses anywhere.
How often does this happen? When did it begin happening?
Once I got the pet. July 21, 18
Steps to make it happen again
Equip the Minishroom Pet.
Your Death Cap gives x 2,25 to the Maester of Magic Bonus(which only gives 1 without the pet). So 1 x 2,25 = 2,25 (Rounded 2). It means that all your troops is getting an additional 2 magic each in this team. The multiply only counts for the Bonus (Maester of Magic). Hope this help.
Also, pets does not need to be equipped to give an increasing Bonus to a given team. As long as you own the pet the additional Bonus count.