it may have to do with transparency.
before you save it, select "merge background transparency" from Objects menu.
(note this will permanently merge the transparency and you wont be able to edit the mask anymore),
but it's worth a test.
ive been thinking if i shouldnt make an option for whether you want to save transparency info in the file or else merge it in the saved file.
let me know if the test solves your problem, it will help the decision making.