Need advice – puppy is picking on my fiancee

Hey guys, I really need your advice on this please.

Our puppy is now 3 months old. We had her since she was 8 weeks. She poops and pees outside, has a routine of bed time, happy in her crate (although whines a bit sometimes), has enough chewing time, sniffing games, etc. We have a trainer working with us so we’re grateful for the support.

Now, here is the issue. She constantly picks on my fiancee. She barks, nibbles, bites her. She also listens to her less during training sessions. Bear in mind she doesn’t do anything like this to me. Obviously when she is overstimulated after being awake for quite some time she does this to me although very rarely and she happily will be in her crate to nap. I’m not sure what to do as this is affecting my fiancee’s relationship with our puppy.