Partners female friend acted different when I met her, but he says everything is platonic?

Please help me to understand this. I met my partners female friend at a works event. When I was there, she was right next to him however she barely chatted to him for the entire time.

Since then, I’ve overheard them on the phone when they are discussing work topics. They chat for a few minutes after talking about the serious stuff related to work. Tonight they were discussing a woman at work they both dislike.

I just want to know why, if everything is platonic, she acted different when I was present? Maybe she was shy or uncomfortable? Because I’ve met my partners other female friends from work and they acted as they would usually with him

Partners female friend acted different when I met her, but he says everything is platonic?
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