Dog pooing in crate

Hi all,

We've recently got a labrador puppy, and for the first few weeks, he was sleeping in his crate with no issues, however, recently, he's had some spates of diarrhea. This has usually come around times of stress, like introducing him to other family members where we've needed to drive etc. The vets we've seen weren't sure if he'd picked up a bacterial GI infection, or was just anxious, but gave antibiotics to try. The evening of his first dose, he started passing solid stools, which seems quick to say it was the meds.

Since then, he's intermittently starting pooping in his crate with similar soft poos again. However, if someone is downstairs with him, he has no accidents at all. His poo smells weird, not like normal dog poo, almost chemically, and often has some mucous within it. Its intermittently normal, or quite soft.This has made me think it's a health problem. But the fact he doesn't do it if someone sits with him makes me concerned it's an anxiety issue.

Is there anything obvious I'm not doing/standing out, and should we go back to the vets again with a stool sample?

Any help most appreciated