I posted above and finally received a letter from the IRS in Jan that it was resolved (this goes back to a sale in Dec 2021). I also used Mason and the initial letter said the Form 8288-B was rejected because the transferor didn't follow procedures outlined in the Code of Fed. Reg. 1.1445.
Along the way, I had a few conversations with IRS agents over the phone (with multiple transfers and 1hr+ waits) and it wasn't until I talked to someone in foreign accounts that I made progress. By the way, this agent was used to dealing with multi-million dollar international account and estate transfers and said this was filed wrong by someone on the IRS side (although it's a tad strange to me we all used Mason ¯\_(ツ)_/¯ ). This conversation was last ~May. He provided me with a new reference number and told me to ignore any warning letters I received later as it would take some time to receive final approval. That was quite difficult, tbh. I never received a seizure of assets notice, however.
Received that final* IRS letter in Jan 2024 (about 2 years after the purchase). It just said that my issue had been resolved. It didn't say what issue, I just assume (*hope*) it's referring to this one as I've had no other "issues" with the IRS.