When importing an SVG to edit, any groups containing only one item are removed. Even though this may make sense in order to simplify the layers, sometimes the groups are important to keep. This is true when creating custom SFSymbols from the exported template. Removing the groups invalidates the template and therefore the symbol cannot be edited with Vectornator at all.
macOS Ventura 13.3.1 (a)
Vectornator 4.13.7