I have encountered that before. I remember a kid I went to school with, his family had come from the UK and he had a noticeable accent when speaking to his parents and sister but his accent changed whenever he spoke to us yanks.

That made sense to me and easily understood why he sounded that way depending on who he spoke to.

This coworker…it’s the same group of people each day she speaks to us and none of us have anything but the standard American accent.