Keep your own interview attire professional even if one interviewer dresses casually. Their clothing is not necessarily guidance for candidates.

Employees may join from home or between meetings, and recruiting expectations can still be more formal for candidates.

A T-shirt in one first round does not mean a partner in the next round will have the same expectation.

Follow any recruiter instructions first. Otherwise, choose clean professional clothing that looks natural on camera.

I would stay formal. Their T-shirt isn't your dress code.