I thought it might be my setting to use the finger as the color picker, so I changed that to "Normal", but the problem persists even after restarting.
It is very streaky. It worked 5 out of 6 times and then it didn't work for 6 times straight.
I then suspected my long press delay setting. I changed it from 0 to 2 and now the reference window works correctly. I can even go down to 0.2 and possibly 0.1.
*shrug* Maybe this needs a fix or just a warning.
I'm on Artstudio Pro 2.1.3 on iOS 13.1.1 on iPad Pro 12.9 2nd gen.