I've already reported bug with mask handling some time ago and it was fixed, but here is another one. I believe it's at least since Odesa version, but not sure, I've used Pixelmator sparingly during summer.
In short - again mask is not handled properly after rotating the layer.
How to reproduce:
- create new project;
- draw something on the layer;
- add mask;
- draw a pattern on the mask with black color, to hide some part of the layer;
- select the layer and rotate it;
- everything seems to be OK, but now:
- select the mask and start drawing - now part of the layer suddenly disappears - it seems to me as if the mask was not rotated although after rotation the image was displayed correctly.
Please stop making bells and whistles and implement quality control. Masks are core functionality in graphics programs, especially the ones that call themselves "Pro".