Okay, so, I unintentionally solved the issue.
Well, I didn’t solve it. More like, figured out the root cause.
It’s similar to one of the posts @AhabGreybeard shared, but not quite the same.
The problem:
My tablet’s pen, as is the standard, has two press keys. I’ve mapped these keys to E (Eraser) and RMB (the pen menu).
The problem was that the button mapped to E was dislodging in it’s hole(?) when I pressed it.
All functions that cannot work when the button was held (like RMB [the other pen button] and scroll) would just not work while the button was dislodged.
I tried using the button in Notepad, but it didn’t create a string of EEEE as one would expect when the button is held down.
So it’s more likely that something is loose inside my pen buttons and that looseness keeps toggling (for lack of a better word) itself when I hit the button. It doesn’t always happen, hence why it was hard to track down.
When I press the E key on my keyboard the issue gets resolved with 100% success. With my pen, it’s less consistent. Sometimes a single click solves it. Other times I gotta spam it a bit.
I tried mapping the button to Ctrl+Z instead of E and the issue remains. So it’s not unique to the E key alone.
My solutions:
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Spam my pen button until it works again (probably not healthy for the pen).
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Bind E to a tablet press key (I’m going with this one. I have two tablet keys I never use and I can bind one of them to E).
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Use the E on the keyboard.
Thank you to @sooz and Ahab for helping me out. Even though the matter had nothing to do with background programs, drivers etc., you guys suggesting it allowed me to rule it out as a possibility, making it easier to find the actual cause.
Hope this helps others who have the same issue in the future!
Edit: in the original post, I said LMB. I meant RMB.