Why does Impress lose track of custom Bitmap fills?

I’ve developed a set of custom fill Colors, Gradients, and Bitmaps for technical documentation. The bitmaps are
very simple, alternating light and dark. They are used to highlight sections of program code in Yellow, Green, or Blue. Earlier versions of Impress seemed to handle them routinely. Now (version 4.4) the tool randomly changes all of
the bitmaps in a .odp file back to Yellow (or whatever the most common bitmap happens to be).

Is there any fix for this? I’ve even tried saving the configuration file, but it doesn’t seem to help.