This appears to be specific to opening pages via Cmd+P.
Most block-level keyboard shortcuts stop working after pressing Cmd+P then Shift+Enter or Shift+Option+Enter to open a page in preview or side panel.
Examples of block-level keyboard shortcuts: Cmd+Z, Backspace to delete a block, Cmd+A to select all blocks, Cmd+Shift+E.
Keyboard shortcuts that target inline text still work (e.g. Cmd+X while selecting a line of text, NOT a whole block). Some other global shortcuts like Cmd+R also still work.
All keyboard shortcuts start working again after I open Cmd+P again and close it.