I was playing with creating a Hausa variant of the Eng glyph. So, I created a glyph named: Eng.loclHAU
When automatically generating features, the locl feature had the correct subs, but there was no languagesystems prefix created.
For now, I’ve manually added the following to the prefix.
languagesystem latn HAU;
This was in Glyphs 1.3.17. fyi.