Okay so I figured out at least one issue with the notifications. Not solved, mind you - I just realized what the issue is.
I had a power user ask why they could not see one of the notifications they created. i logged in as that user & recorded all of the notifications they could see. Then I looked at the notifications from my own account & searched for the items that they should be able to see. After opening each one - both those the user could see & the ones they no longer could see - I determined that they no longer have visibility of the notifications that I &/or my fellow SuperAdmin had to alter. Which, given the amount of effort we had to put into deactivating those with inadequate filters, was a lot.
Now that I have it figured out, I’ll submit a ticket, but i wanted to share this w/ everyone so they know what to look forward to.