The bug

If a pack is selected from the Available list or deselected from the Selected list in the pack menu and then the game is switched from windowed mode to full screen or vice versa, the game reverts the changes that have been made. This is not the case in 1.16.1 and older versions.

The bug is caused by the fix of MC-188473.

How to reproduce

Before reproducing the bug using either method, open the "Data Packs" or "Resource Packs" menu.

Selecting a pack

1. Drag and drop a pack into the menu if there is nothing yet, and then select it.
2. Press F11 to switch to full screen or windowed mode.
   → The pack is deselected again.

Deselecting a pack

1. Choose a pack to deselect.
2. Press F11 to change the display mode.
   → The pack is selected again.