Sorry to hear you're getting the runaround from PayPal. Evidently their right hand doesn't know what their left hand is doing, so every MTS ticket anyone opens about it is getting treated as a separate, isolated issue with little if any context relating it to other pertinent tickets, rather than instances of a single, common issue at their end.
At least that "Jeremy" seemed to be aware of a "callback URL" issue they'd mentioned in a related ticket, which we'd already responded to as a red herring -- they were saying our callback URL didn't return any shipping options, but that was a case where the seller they'd tested with doesn't even sell any physical goods that require shipping, so of course no shipping rates were returned in that scenario, where (until this issue arose) they'd heretofore always applied the precalculated tax amount we passed to checkout in the first place.
I don't know if responding to your own MTS ticket would do any good, but if you do, it may help to reference our own MTS case # 10167138, where PayPal has confirmed the issue at their end that they're working to fix and been escalated at their end.