Can't use CMD+V Shortcut to copy paste into certain apps, but right click menu works fine
With help from @user3052786 , I've found the issue.

The reason why many (but not all) apps were failing to paste with the shortcut (but Edit menu -> Paste or Right click -> Paste worked fine) is because I previously followed the following tutorial on how to make my clipboard content attempt to match the output document's text style on paste.

https://havecamerawilltravel.com/photographer/set-paste-match-style-default-mac-osx/

I did this because I was using OneNote for Mac and pasting a lot of content between different documents for note-taking.

The fix: Simply remove the 'CMD+V -> Paste and Match Style' mapping from Keyboard shortcuts in MacOS Settings, as shown in reverse by the tutorial.