I should also note that the core issue may have been that some fields in the payment session that were numbers/int were incorrectly being typed as strings. Specifically, epochTimestamp and expiresAt. While this may have been the specific issue, the solution was to simply not bother deserializing the payment session into a type and simply forward it to the frontend and, ultimately, to the Apple Pay SDK.

Update: My team was able to resolve this issue! It turns out that our server was trying to type the opaque payment session response from Apple, instead of treating it like an opaque string and simply forwarding that raw string response down to the frontend, to be sent into completeMerchantValidation().