I think I found a bug.
These are my first attempts to get a nebula effect. (At this point it just looks like a purple blob.) The nebula attempt is in it's own layer of course, the bump blend mode is set to replace. It has a very low opacity. The nebula is done with the paintbrush tool. The starfield is made with the glitter tool.
The bug: Notice the black squares in the starfield that has formed around the Nebula Attempt? And that you can't see any stars through the nebula despite the nebula layer being at 4% opacity? I don't know why the squares appeared, or why they survived the export to JPG. If I erase the nebula, or just hide that layer, the starfield goes back to normal.