Some glyphs dropped

I'm making a mainly monoline font and one of my masters has open paths. I don't use this master for interpolation, but just having the master in the .glyphs file means that some but not all glyphs become empty. (By empty I mean the outline is just`0 endchar`) If I remove the master and save as a separate .glyphs file, the glyphs convert just fine.

I am not sure why it's certain glyphs but not others.