Ironically, I could’ve read all 50 pages of that RFC and still missed the actual issue. What really helped was RFC 1331[0], specifically the "Async-Control-Character-Map" section.

That said, I’m building a product - not a PPP driver - so the quicker I can fix the problem and move on, the better.

[0] https://datatracker.ietf.org/doc/html/rfc1331