Just tested the original cart - same thing
I'm finding references at PayPal.com that would indicate we have to do something different but I'm encounting bad links when trying to investigate.
It's definitely something to do with this:
Calls SetMobileCheckout API to initiate transaction.
The API sets a token, which is appended to the PayPal payment URL.
Seems like this is something that WebAssist would want to look into.