One of the 4 brush alphas of the initially selected brush is a square (next to lock alpha). That is a directional square brush. Squares are rotated according to stroke direction. That makes the bg pretty easy (square brush with ~20% opacity).

If you can reliably reproduce the bug you've observed with just a few steps I would be infinitely thankful. Regardless I'm on it.

Edit:
this visual is probably it better than my explanation https://i.imgur.com/xvHOMhu.jpg

Just tried a few things but can’t hit the bug now. As an aside a different bug I noticed: the app would miss taps with the Apple Pencil on the toolbars almost every time after drawing a stroke.

Hmm I tried to reproduce the missed taps, but they do seem to register for me. Apple pencil 1st gen, ipad 7th gen 2019, ipad os 13.5.1
Luckily I discovered another bug. lol

it doesn’t seem to happen immediately, but after drawing a bit—might be related to rotation/zoom with multitouch. I’ve also noticed undo is semi broken in this state (I can’t undo/redo everything). I’m using a 2018 ipad pro with pencil 2, iOS 14.4 using a smart keyboard, so pretty different configuration. I’ll see if I can reproduce it more readily

Thanks a lot for testing! It's going to be a busy week for me haha

I really like this piece!

nice style