When font shorthand is specified all of its subproperties including font-size-adjust, font-kerning, font-feature-settings, font-optical-sizing and font-variation-settings should be reset to their initial value.
Firefox: Shipped/Shipping — Gecko is already resetting all the required font subproperties, shown in the link above,
font-synthesis-* should be "Cascaded Independently" according to spec: https://github.com/w3c/csswg-drafts/issues/7832#issuecomment-1338671264
Also tested in Firefox stable, the feature is working.
Safari: Shipped/Shipping — The feature was recently shipped in Safari Technology Preview 160, not yet in Safari.