This (https://github.com/adobe-fonts/source-sans-pro/blob/master/Roman/family.fea#L621) line
sub [i.trk germandbls iogonek.d] by [Idotaccent.sc Germandbls.sc Iogonek.sc];
needs to become
sub [i.trk germandbls iogonek.d] by [Idotaccent.sc germandbls.sc Iogonek.sc];
InDesign still does the right thing, but not all editors might.
This (https://github.com/adobe-fonts/source-sans-pro/blob/master/Roman/family.fea#L621) line
sub [i.trk germandbls iogonek.d] by [Idotaccent.sc Germandbls.sc Iogonek.sc];needs to become
sub [i.trk germandbls iogonek.d] by [Idotaccent.sc germandbls.sc Iogonek.sc];InDesign still does the right thing, but not all editors might.