Good afternoon. I'm writing through an interpreter, so sorry for the mistakes.

I use the XP-PEN Deco 3 tablet. When drawing, when the brush leaves the canvas to select the color and when returning back, the brush itself disappears and the cursor appears in its place as in the screenshot, sometimes an ordinary arrow. However, it does not draw until you select another brush. This manifests itself very often, it is impossible to work.

Krita 4.0.1
Tablet XP-PEN Deco 3
iMac 5K 2017, Intel Core i5, Radeon Pro 575

Sounds like https://bugs.kde.org/show_bug.cgi?id=390642 -- I've seen this as well, though not frequently, but then I don't use macOS frequently. Nobody who actually knows anything about macOS development has stepped up to help investigating this problem by getting, building and debugging Krita's source code, so there's no solution yet.

The problem is similar, but the brush does not draw at all after selecting the color and hovering the cursor. On Linux, Krita showed herself with the best of luck, and here such a trifle and such a huge problem. In any case, I will look for a solution to the problem. Thank you for your reply.

It's very much a macOS-only problem, it seems. Might be the OS, might be the new version of Qt we're using, might even be a bug in Krita...