(old thread, I know)
I have a Pro 10.5" I have a similar problem with. I plug a Pencil (Gen 1) into it, pairs the first time, shows maybe 3% battery, then pops to 100% after a few minutes. It works for maybe 30 seconds. After this, it intermittently shows up in Bluetooth "My Devices" for pairing but will never actually connect when clicked on. I recently got another refurbished Pencil 1 from Apple that behaved exactly the same way. Even charged it with a USB-C to Lightning adapter outside of the iPad. Eventually both of these just refused to work, no battery level, stops even showing up in Bluetooth.
I'm beginning to think it's something about this iPad and something that happens to them after being plugged in for a while. Frankly, I'm afraid to plug yet another Pencil 1 into it now. Charging and other accessories work fine with this tablet however.
If I bother to get another Pencil, I'll try it on an iPad Mini I have first. Oh and both of these Pencil 1's I've tried with this newer iPad Mini and they wouldn't work/pair there either. These Gen 1 Pencils are just bad IMO. :/