Sometimes when Glyphs generates lookup for Arabic, it adds RightToLeft to lookup flags. This is however is both unnecessary and misleading. The only lookups where this flag make a difference is cursive attachment lookups and has no relation to text direction. From the spec:
This bit relates only to the correct processing of the cursive attachment lookup type (GPOS lookup type 3). When this bit is set, the last glyph in a given sequence to which the cursive attachment lookup is applied, will be positioned on the baseline.
Note: Setting of this bit is not intended to be used by operating systems or applications to determine text direction.
I suggest that Glyphs stops adding this flag to Arabic lookups as it just confuses people thinking it makes a difference.