I’m building a variable font with virtual masters (it is quite possible I got the setup wrong). The glyph outlines variate but mark positioning is not (the three strokes should be attached to the right):
What version do you have? And can you send me the file?
3.0.3 (3081). Sent the file to support email, test string is مِثلِهِ.
It is probably because of the fact that the stretch axis is made with brace layers and not a full master. The algorithm is not good with that. We are about to change the whole stack that should fix this. I’ll see if I can improve it until then.