This is fine, a capacitance from VSS and VDD to ground play or to infinity - that becomes capacitance to net "0".
I actually always recommend to companies to use artificial net "0" as a ground - this way, you can easily separate capacitance form nets to VSS and to VDD (to real metals) vs capacitance to the ground plane.
Self-capacitace, i.e. capacitance form VSS to VSS is not a nonsense (although you normally do not see this in extracted netlists).
One reason is - coupling between metals of the same net, that have high resistance (e.g. long narrow metal line) between them.
Or capacitance from VSS to groudplane - and when you declare VSS as a ground, this becomes capacitance VSS to VSS.
Intricacies of parasitic extraction, that are...