Have you recently check for Windows Updates and video driver updates?
If you could provide a log file when the issue is happening?
There is a section in the logfiles that shows how OBS is resizing the window (example below)
output 0: pos={0, 0}, size={1920, 1080}, attached=true, refresh=60, name=BenQ GW2480
output 1: pos={-1366, 304}, size={1366, 768}, attached=true, refresh=60, name=