According to this site (& others)
Autotrader - page unavailable (link works, page down to bottom)
in CA vehicle plates transfer when a car is sold. In my state (NC) that's not true. Here, although annual replacement "sticker" fees are required, as long as it's in good shape and remains readable, the plate itself can be reused for up to 10 years before
mandatory replacement. It's not a choice to replace it after 10 years-- the annual registration letter makes that quite clear! And it's not 10 yrs on the new car, but 10 years total new and old car(s) usage combined.
In CA the plate goes with the sold car. So when HP sold each of the 3 Porsches she supposed owned, a different plate should have gone with each of the sales. (Even with tag transfer, that's one reason it's expensive to buy and sell cars frequently. New registrations cost money even with existing tags.) Anyway, none of the Porsche plates could legally be put on the RR.
The only exception in CA is for specialty or personalized plates as they those belong to the owner.
So if a plate photographed on the RR truly traces back to a Porsche, and it wasn't a personalized or specialty plate, it surely sounds like something hinky was done per CA law. OTOH, if the plate number was located in non-DMV database and was associated with Heidi at one point and now tracks back associated with a Porsche, that makes perfect sense. It's on a Porsche HP sold.
I can't imagine a legit reason to photograph the RR with the plate from another car. It's not a legal or even quasi-legal "short-cut" because the RR couldn't be registered to her with the wrong plate. (If the used RR came from a dealer, usually the dealer does the work of transferring stuff, at least in my state. They charge a fee for that but I don't think the fee is optional for a dealer.) And the owners of the new Porsche likely would have a heck of a time registering their used Porsche without the plate it was supposed to come with. So I'd think the plate on the RR was its plate at the time of the photograph. BUT that plate could have been replaced since the photo was taken if the old plate became unreadable or was stolen or lost. HP also could have changed a personalized plate to a standard one or vice versa.
JMO