I was able to miraculously solve this problem using the "try a bunch of random stuff" approach. The fix turned out to be this... even though the Stripe documentation said:

const meterEvent = await stripe.v2.billing.meterEvents.create({

I ended up trying it like this:

const meterEvent = await stripe.billing.meterEvents.create({

and THAT was the fix.

In other words the error message Stripe returned had nothing to do with the root cause of why it wasn't working in my case.