for me, when I hit the x to exit Streamfab, it closes. Nothing listed in task manager for it. Occasionally CEF takes a minute or two to close down when exiting SF.
If all else fails, run these two commands as admin and see if either finds/fixes issues:
sfc /scannow
dism /Online /Cleanup-Image /RestoreHealth
Ideally you want SFC to find no integrity violations, and DISM to just report that the operation completed successfully.
Note: DISM can sometimes appear to hang (progress bar stops) in the 60-70% range, this is normal even when you don’t have issues.