What output resolution is the Pi producing when driving those two displays?
If it’s sending different resolutions (e.g. 720p to the projector but 1080p to the monitor) then it may be the same issue from this thread:
What output resolution is the Pi producing when driving those two displays?
If it’s sending different resolutions (e.g. 720p to the projector but 1080p to the monitor) then it may be the same issue from this thread: