Please try this workaround:
1. Open Action and select Screen Mode
2. Add game executable name (like csgo.exe etc.) to excluded process list using Settings->Excluded process->Add (you need to find csgo.exe location in steamapps folder)
3. Start the game in fullscreen windowed mode
4. Scroll down to see "Application:" in Action (at the bottom of main view). Click select application. A window picker will popup. Select your game window from this picker (the game must not be minimized)
5. Check if game is visible on preview
6. Record/stream whatever. Only the game will be captured and no other windows that can pop over it as it can happen in fullscreen windowed mode
7. You should get even better performance and less frame drop than in game mode
(we have tested it and usually get zero frame drop)
PS. This only works on Windows 10 version 1909 or newer