@gstrauss provided the following feedback on https://lists.w3.org/Archives/Public/ietf-http-wg/2025JulSep/0094.html wrt 4.3.2

"MUST include the length in the Upload-Length header field if known"
Over-specified. If it is okay to exclude it if not known, then this should be SHOULD, not MUST.

Stating how the client deals with a missing field probably would not go amiss either. Borderline editorial/minor design change.