Using the Google font Cairo Variable, in this sequence, "فَعَّل," the medial ain will not connect to the lam. The shade (0651 + 0643) intervenes and sets in the wrong position.

To Reproduce
In InDesign (perhaps other programs as well), key or copy/paste the sequence with Cairo variable loaded and applied. Then Unload Cairo variable and load Cairo Regular static to see what the corrected sequence.

Expected behavior
The shadda should be centered above the ain.

I have contacted the font manufacturer, and the font behaves as expected in their tests. I also tested the font myself in Illustrator, and the sequence is correct with both the variable and static fonts.