Custom SF Symbol gets error "not interpolatable"

I'm running in to the same issue using Figma and creating a custom icon. I try to create the different weights by adding a stroke to the custom icon and then outlining the stroke and flattening the icon. If I just add the icon in to the different weight groups without editing it then it works fine and validates. It's once I start editing the icon and try to make it thicker or thinner that I think it doesn't work. If you figure out what the solution is let me know.

From my experiences so far, there needs to be a one-to-one relationship between vertices with the three weights. This way the engine can match up where to interpolate between the other weights. I'm still experimenting myself, but I wish there was an obvious way to manually set this relationship.

I seem to find success if I simply scale a path to make the shape appear thicker or thinner (command-K in Sketch). However, this does not always give a good result compared to say, "offset" of a path.

Adding a "me too" here. Illustrator doesn't seem to handle floating point math particularly well, as you can edit an svg, save that svg, close the svg and reopen it, then just toggle a layer visibility on and off again, save again, and the svg's paths will all have magically changed. The result: a previously working SVG will no longer be exportable into SF Symbols due to some random new points added to a random subset of curves :(

I've yet to find a reasonable solution to this yet sadly, but I suspect it has to do with how floating point math and path computing is handled in SVG editors.

One thing that seems to help is, both before and after you generate a new offset stroke, click the "Convert selected points to smooth" button one very anchor point that's meant to be smooth. This often results in a slight tuning of the anchor points, which causes offset strokes to be less error prone and more likely to stay up-to-date across file saves/loads