Icon lookup machinery ruins working on system icons
This isn't an issue of Symbolic preview itself, I've ran into a similar issue with fullcolor icons trying to use App icon preview for fullcolor variants of device icons.
When you create adjustments of a system icon, such as anything from core.svg, Symbolic preview will always use the runtime version of the icon, not the asset from the SVG sheet. The only way to see the new symbol is to rename it. Luckily the export itself works from the SVG sheet.