As Martin Hosken reported in https://github.com/fonttools/fonttools/pull/1096, feaLib currently incorrectly handles the case where a marked input glyph sequence in a contextual chaining sub/pos rule is split into multiple runs, rather than being a single continuous run of ' marked glyphs. The consensus there was to raise a syntax error like makeotf instead of second-guessing and silently fixing it like fontforge does.