Probably your icon consists of several layers of vector paths. Try to flatten (Cmd/Ctrl + E) these layers into one.
And using variants for different icons is not the best way.
Figma note: We don’t recommend using variants to group different icons.If you have different sizes of the same icon, then you could combine these as variants instead.
https://help.figma.com/hc/en-us/articles/360056440594-Create-and-use-variants