This is apparently a long-time issue with Unity. One website suggests checking the "Disable Fullscreen Optimizations Box"
1. Click the settings cog next to Launch
2. Click Manage Game
3. Click Open folder in Explorer
4. Open the AmongUs folder
5. Right-click Among Us.exe and click Properties
6. In the compatibility tab, check the Disable Fullscreen Optimizations checkbox.
Hope that helps!