In Text Preview, when I try to activate stylistic set 19 for a font (in this case, Matthew Butterick’s Concourse OT 3 Book), the preview does not show the substitution. I can activate this stylistic set in apps that support OpenType features, and I can preview text with stylistic set 19 activated in another font editor. Text Preview appears to correctly show stylistic sets 1-18. See images below, and please ignore the background colour issues.
Maybe the Q glyph already got replaced by one of the other activated features? It looks like even without the ss19 feature active, the Q does not have the standard form in your first screenshot.
Hi Florian, and thank you for your response. This is indeed the case. Apologies for the lapse on my part. 