Mutate related. Bug or intended? [Investigating]

PChicken

I have two creature card with "Mutate" in a deck. "Sawtusk Demolisher" and "Migratory Greathorn".
When one of them are mutated on the other, if there are no opponent support on field, "Migratory Greathorn" 's "Mutation: ...." effect is not triggered.  It seems like to be blocked by  "Sawtusk Demolisher" unfulfilled trigger condition.

So, is it a bug? 
Or there is a rule that: 
When a creature with "Mutate" mutate on other creature with "Mutate" , if either one of the "Mutation: ...." effect cannot trigger, another effect don't trigger.

Magic:PQ Support Team

Thank you for the feedback!

This issue is now under investigation.