Cheers Todd, it’s a bit of a clunky process, involving GIMP at the moment, but I can now actually get complex subject selections etc from Photoshop into Darktable.
All masks are modified in processing modules if there is distortion, cropping, rotation or orientation. So this is working exactly as designed i would say.
Specifically for the example in the initial post:
Could it be that the base image has a rotation in the metadata, and the mask doesn’t (as it is saved in the correct orentation)?
Reasoning: mask (in portrait orientation!) is loaded and stretched to fill the image (natively in landscape orientation), then both are rotated acording to the EXIF information