It might, but in a very high proportion of cases the networking stack takes the happy path and a single logical protocol unit translates directly to a single pa

It might, but in a very high proportion of cases the networking stack takes the happy path and a single logical protocol unit translates directly to a single packet on the wire. That being the practical reality, it's quite possible to write a stateless protocol parser and have it apparently work. Except, as you point out, when it doesn't.