I have a base glyph with an anchor called
gaf and a mark glyph with an anchor called
_gaf, and I would expect the mark to attach to the base. It works in fontmake, but not in Glyphs export.
What seems to be happening is that the mark attachment code is generated, but it has a mark filtering set which does not include the mark glyph. This seems to be related to the use of contextual anchors; although this particular mark attachment does not use a contextual anchor (it’s just a normal anchor attachment), other contextual anchors use a mark filtering set, and that particular set is wrongly being applied to this particular attachment.
This is with Glyphs 3.2 (3195).