ufo2ft: RTL kerning missing from font

This is a strange one, but I don’t know how to help myself any further:

A font of mine doesn’t generate RTL kerning (or at least it doesn’t show in both FontGoggles and Indesign), while LTR kerning generates fine.

ufo2ft code base is up-to-date with the repo.

I’ve had fontmake export the debug feature file and pasted all the code relevant to the kerning pair راbelow (ر is supposed to be first from right, ا is second):

languagesystem DFLT dflt; # Default, Default
languagesystem arab dflt; # Arabic, Default

@kern1.reh = [reh-ar ...];
@kern2.alef = [alef-ar ...];

lookup kern_Arab {
    lookupflag IgnoreMarks;
    pos @kern1.reh @kern2.alef <-100 0 -100 0>;
} kern_Arab;

feature kern {
    script arab;
    language dflt;
    lookup kern_Arab;
} kern;

Looks correct, doesn’t it? Any idea where to look further? Thank you.