I have same issue. It's a Zoom problem. It's the only app I have that won't respect the change in camera choice when two cameras appear to have the same name in the picker dropdown (Studio Display Camera).
The workaround is a little bit of a hassle, but it works. Disconnect the undesired monitor, start Zoom, go to settings/video, ensure it's using the correct camera, reconnect monitor. You may have to reselect the correct camera again in Zoom settings but it should now keep the proper camera. At least for a while.
Every other app I use will respect the camera selections (WebEx, Teams, FaceTime)
BTW, I have reported the bug to Zoom, so we'll see.