If the PIA app appears transparent or partially invisible, it may be caused by Windows' Transparency effects setting. Toggling this setting off and back on often resolves the issue.
Toggle Transparency Effects
Open the Start Menu and select Settings.
Go to Personalization > Colors.
In the Transparency effects field, turn the toggle off.
If the app still appears transparent, try turning Transparency effects back On, then restart the PIA app or restart your computer.