I just drew a bonus hero which gave me 4* rocket n groot. But guess what? 4* rng is not in any of the heroes I favourited.
Do you have any 4* characters marked favourite? When the game decides to give you a bonus hero reward, it will always be of the same rarity as the cover you first receive. So you’ll never receive a 5* bonus hero off the back of pulling a 3* cover, for instance.
If you don’t have any characters of a particular rarity marked favourite, the game will pick a random one for you. Does that sound like what you experienced?
You can read more info about the system here:
https://d3publisher.helpshift.com/a/marvel-puzzle-quest/?s=gameplay-tips&f=what-is-bonus-heroes&l=en
A person in my alliance had the same problem. When he switched to having only one hero in each tier selected everything then worked fine (he always got that hero). Maybe try that?