Question: feature file copying behavior when running from designspace document?

Imagine a bunch of master UFOs and a Designspace document, only one master has a features.fea file and this master is designated ``. Also imagine the feature file implements a custom GDEF table. Trying to make a VF can now fail at the varLib merger stage with mysterious failed assertions if the custom GDEF and the auto-generated state for the other UFOs disagree (I spend 4-5 hours yesterday debugging this, turns out the designer forgot to add one alternate glyph to `@BASE`).

fontmake/ufo2ft currently seem to compile each source separately without caring about `copyFeatures`. https://github.com/fonttools/fonttools/tree/master/Doc/source/designspaceLib#sourcedescriptor-object says `copyFeatures` is only applicable to MutatorMath. Yes, in this instance, I could (and have) fixed this by simply including the master feature file everywhere.

My question: if you run from a Designspace document and you have one source with ``, shouldn't that feature file be used for *all compilations*? Instances UFOs, master OT binaries, instance OT binaries?