I think I figured it out. I tested exporting a different template (the pencil icon) and I did not have the same issue.

Hello, did you ever find a solution to this? I am having the same issue. I didn't even create a new icon. All I have to do is download the symbol template, open and save it in Adobe Illustrator with no changes, and try to validate in SF Symbols. I still get the error message. I even tried Inkscape and got the same interpolation error message.