From an API perspective I agree with you.
From a UX perspective I agree with them.
In the end, you're writing software to benefit users, so user benefit is top priority.
Luckily, you can have it both ways. Keep the API pure and simple, returning a meaningful error to the client, and the client then procures a new JWT completely transparently to the user, and retries.
I suspect you're overcomplicating things by using a JWT, but that kind of decision screams "confounding factors" that affect design decisions that you haven't/can't elaborate on. It'd just take some minor tweaks of the standard "shopping cart" API/DB design to get what you want, so I assume there is a reason you haven't gone that route.