2017 iPad Pro battery dropping from 100% to 2% WHILST CONNECTED TO POWER

(Apologies for the caps, but felt I needed to emphasise that last bit)

I have a 2017 iPad Pro, running iPadOS 14.6. Just recently - and I can’t recall when I first noticed it - the battery indicators (that is both in the top right corner and the widget) will suddenly show the battery to be 2%.  It will literally be 100% one minute, and then I’ll leave it to go to sleep and, the next time I look at it, it’ll be 2%. 

I’ve tried replacing the power hub from a no-name to Anker, and replacing the cable (it was using an Amazon Basics cable and, in my experience, they’re false economy. I have replaced it with an OtterBox cable). It was fine for 2 or 3 days but it’s happened again just now (indicator now stuck on 4%). I’ve even tried plugging it directly into the wall, but that didn’t help. 

Can anyone shed any light on what might be causing this, as I really can’t afford to replace it right now, and it’s in constant daily use. Is this a sign the battery is dying…? How do I check it…? If so, will Apple replace it…? I spoke to my parents, who have a first generation iPad Air, which is obviously far older than this, and the battery in that seems to be fine.

It’s now refusing to budge past 4%. Oh it’s now 5%, but it’s glacially slow. 

Any help, folks…?

ETA: It will now go up 2%, then back up 1%, then down 1, then back up 2. It’s back up to 35% from 33. I have no problems with my phone or my MacBook Pro. Just my iPad. My phone is connected to the same hub.