This is clearly an Apple bug. Have either of you raised a Feedback report? Perhaps if it's raised there someone might see it and fix it?
Anyway, as a workaround, you could ask Apple for an address to post a physical form to. If they'll accept a physical form, download it from https://www.irs.gov/pub/irs-pdf/fw8ben.pdf, print and complete it, then post it to them.
If you can afford tracking and/or signature on delivery (ask Apple if they'll sign when they receive mail that requires a signature; some companies won't), pay for it so you know they've received it.
I think - can't remember as it was almost 20 years ago - when I set up my dev account for the App Store I had to post the form, so back in 2008 they accepted them.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect