Ok found the fastest, easiest and 100% so far working way to get it to set to 100% instead of 0%. At least on my device(iPad Pro 12inch 2018)
Open art studio pro and a document, if you haven’t it set already, set 1 finger function in gestures to “adjust brush size/opacity”, go back to the document, drag your finger all the way UP (adjust brush size), and that’s it, the bug switches from 0% to 100%. If you open a new document, you have to do that again. Hope it helps until it’s fixed.