Fonttools always writes gvar tuples for composites glyphs even when deltas are all zeros to support old macOS,
https://github.com/fonttools/fonttools/blob/65bc6105f7aec3478427525d23ddf2e3c8c4b21e/Lib/fontTools/varLib/__init__.py#L296-L314.

fonttools/fonttools#1788 proposes to remove the workaround.

We should implement the workaround, and remove it if/when fonttools does.