I think Glyphs’ devanagari code is incorrectly handling ja-deva + halant + nya-deva + halant + ca-deva. This should be processed into j_ny-deva + ca-deva. With glyphs it is processed into j_ny-deva + ny_ca-deva. See the image, which shows Rhodium, Vesper Libre, Murty Hindi, and Adobe Devanagari. I do not think this is a critical bug, as the ज्ञ्च sequence does not appear in corpora for contemporary Hindi, Marathi, and Nepali.
I think to remember that the general rule was to join from right to left. Is this an exception or are I’m mistaken?
I’ve started a Typedrawers thread about this. I’m assuming the two right forms are correct because of the fonts’ creators, but I might as well be sure.