I recently needed to revert the changes I’d made to a master by getting rid of the master and going back to an old exported instance and using “Add other font” in the masters panel.
When I did that, I found that some of the outlines didn’t import, because the glyph names were subtly different. Export to OTF and then adding as a master doesn’t round trip nicely.
As an example, when you add a Euro sign in Glyphs using the “Symbol > Currency” menu, the glyph is named “euro”. But when Glyphs exports the font, the glyph is renamed “Euro”. When you pull that OTF back into glyphs, it doesn’t get added. Similarly the arrows are called “upArrow”, “leftArrow” etc. when you add them from the Symbol menu, but export as “arrowup”, “arrowleft”.
Could Glyphs become a bit more clever about remapping glyph names when adding a master?