Sadly, I have no other pen enabled iPad. Just for the heck of it, I tried following your advice, but connected to an old iPad Air 2 instead. Didn’t help, but worth a try.
I get the impression, that the newly introduced Bluetooth access permission system somehow gets in the way.
it doesn’t help, that iPadOS can hide pop up menu’s (I.e, touch confirmations being invisible, but if you touch the home button as if the dialog box was visible, touch works anyway - sigh - a huge security problem in the making ;-). There might have been other invisible pop up dialogboxes, besides the visible one asking for pairing.
This new iPadOS is not what I was prepared for. It’s developing into a weird monster, although version 13.1.2 is less of a “frequent dier”, then earlier versions ;-)
The Problem with Apples “everything just works” approach is, that there are no tools to I.e. reset Bluetooth alone (as a test). It’s either the “big hammer” approach or a lot of wasted time fiddling with peripheral settings, that really has no significant effect on anything.
Sigh!