Using multiple svg layers on different masters

I’m making some color fonts from svg files. I’ve got one file with multiple masters, one for each color variation. The masters are duplicates from one another, only changing the Custom value under Proportions (1, 2, 3…). Each of these masters has its own svg layer, which references images from a specific subfolder.

The svg images are displayed correctly within Glyphs, but when I export, all fonts end up using the same svg image. In my current setup, all fonts get the Marble svg files. I’ve tried it on 2.4.4 and 2.5b (1101), same result.

These images will better explain my setup:

I have fixed it.