In the Library module, I select a group of photos and use "compare" mode. Up until I upgraded to 9.4, I could use the arrow keys on the keyboard to move between the pictures in the group. That's broken now. It's not my keyboard (keys work fine). The graphical arrows on the Lightroom interface still work with the mouse.
Updates for the Adobe Photography Products were released on 10.20.2020 that include fixes for these issues. Please install the most recent update and confirm that your issue is now fixed. Please let us know if you encounter any issues.