I don't think this is really an example of Hyrum's law. Hyrum's law claims that even if you carefully document your contract, someone will rely on the observable behaviour rather than the documentation anyway.

But this is an example of a much weaker proposition: if you don't document your contract, then people will guess what the contract is and some of them will guess wrong.

(In fact in this case it seems it's more like "if you don't document your contract and your support staff sometimes say the behaviour is A, people will rely on the behaviour being A".)