I also have this issue, where the volume is forced to 100%, overriding my system settings, however it happens to me not only when I'm joining a call, but when I get a notification about a teams chat message and it doesn't matter if I have Bluetooth headset connected or just use my laptop speakers.
I might be watching a video on 30% volume and hear an ear-piercing "ba-ding". Then I have to immediately turn my system volume down from 100% because my video is way too loud. This is very annoying and I'm sure not beneficial to my auditory health.