Page Up in collections does not work

OK - it's working here on Big Sur.

First thing I'd suggest is to try restarting your mac. If that doesn't work, try restarting in safe mode.

https://support.apple.com/en-gb/guide/mac-help/mh21245/mac

Safe mode will clear temporary files and caches, and sometimes fixes odd glitches such as this.

Otherwise hopefully someone else with Monterey can confirm if it works for them.

(It will take longer to start in safe mode than a normal start)

The fn-△ and fn-▽ keys are working well for me in Photos 7 on macOS 12 Monterey.

But what seems to be broken in Photos 7 is the ability to drag photos in an album across several pages of thumbnails. Photos is no longer automatically scrolling the album when I reach the border of the window when dragging.

What do you mean by collections? Albums?