The glyphs are set to export and Glyphs reports no errors in the the feature section. Issues only appear when you try to export them.
The code is a simple substitution test for ligatures and variant forms using zero width joiners and variant selectors.
sub o-runic zerowidthjoiner kaunK-runic by _o_k.runic;
sub o-runic VS1 by o-runic.reversed;
sub o-runic VS2 by o-runic.inverted;
sub o-runic VS3 by o-runic.turned;