Disappearing Masks in Pixelmator?

Hi, I hope you can help me figure out an issue I'm having with Pixelmator.

First, info: I'm using v3.1 (marble)

Mac OSX 10.9.2 (Mavericks) on a 2011 MBPro (8gb RAM, 2.8GHz intel Core i7)

Plenty of HD space on the MBPro.

Here's the issue:

I'm building a layered photo image made up of a set of jpgs (240dpi, all about 7mb in size). This project is currently 22 layers, all with layer masks (painted black) except for the base exposure image. The current pxm file size is about 530mb.

At about 20 layers, I noticed that suddenly, one or two layers would lose their masks completely. By "lose," I mean the mask would turn white, as if I had filled it in completely. This is really annoying. Even after quitting (w/out saving), and then relaunching, and then opening the file, the white masks are still there.

A screenshot is attached showing the layers.

Is this because the pxm file is too large? I don't have any apps other than iTunes and Mail running when this happens. Or have I reached a "layer limit" in PXM?