Do you have a ducker? In OBS, that would be a Compressor filter on the Mic, with its Sidechain set to the Desktop. You (or someone) would have had to add that; it's not there by default.
Another possibility, if you really don't have any processing in OBS (lots of places it could be), is that Windows is doing that, trying to be smart and helpful, and falling flat on its face like it often does. Take a good full day to look through ALL of Windows' settings, figure out what each one does, and get rid of everything that is not "a straight dumb wire". Several things are on by default, either for "consumer candy" or to make a conference call work in a terrible environment. You don't want any of that.
