Custom brushes mask colour is white

hello, everyone. i couldn’t find anything about this:

i’m trying to get dithered alpha on layer. i use custom brushes and i can use them with eraser just fine.
i wanted to use dither pattern on larger area, so i tried paint bucket tool with foreground colour being mask. it fills the selected area with white colour pattern instead erasing it.
pencil tool coincidentally does the same - it either ignores mask as background colour and ‘paint with background colour’ option, or paints with white when the foreground colour is mask.

is this a bug or do i need to have specific settings when storing custom brushes?

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