I have never seen a modern framework which does not handle HEAD requests transparently as soon as you define a GET request… Which framework are you using?
The idea behind a protocol specification is that docs are not cluttered with all kinds of details. Deviations of the specification should be mentioned, but it’s impossible to document where something relies on the specification. If your framework indeed does not handle HEAD requests, then that would be the place where it should be documented.
I do agree with you that throwing or logging an error would have been very helpful.