Images where the height is not a power of 8 will be typically padded.
(The dimensions of the image are encoded so I think the value of the padding won’t make a difference because the renderer will skip the padded area, but I’m not completely sure about that.)
It’s likely that your mask is the wrong way round or the padding is being padded with ‘solid’ rather than ‘transparent’.
In most siuations, a bit mask uses 0 (black) for ‘solid’ and 1 (white) for ‘transparent’,
but for whatever reason it’s the reverse on the Arduboy,
1 (white) for ‘solid’ and 0 (black) for ‘transparent’.
Oh, and if you’re using an external mask,
the external mask doesn’t need its dimensions encoded at the top, it pulls them from the non-mask image.
Leaving the dimensions in the mask would cause the mask to be corrupted slightly, so they’ll need to be removed.