Add Other Font doesn't bring all glyphs with it

I’m trying to add files for separate weights into a single file. The problem is that if the font I’m adding has glyphs that aren’t in the current file then they don’t get added at all. For example, the Black has a few extra glyphs for contextual substitutions and those glyphs just disappear when I use the Regular and go to Font Info > Masters > Add Other Font.
I didn’t notice at first, but then export failed because the features refer to those glyphs and they had disappeared. I think by default any glyphs not in the current font should be added if you do Add Other Font as a master. Or at least give an option or a warning or something.

Add Other Font adds only layers to the glyphs that are present in the primary font.

You could use the Compare Fonts command to sync glyph sets first.