@gstrauss provided the following (slightly edited) feedback on https://lists.w3.org/Archives/Public/ietf-http-wg/2025JulSep/0094.html wrt Section 4.2.2

"These interim responses MUST NOT include the Location header field"
Over-specified. MUST NOT should be SHOULD NOT, and/or the doc should say that Location should not be included in any 104 response other than in the initial 104 response when the upload resource is created, and that a client SHOULD ignore Location in any 104 response besides the initial 104 response for the upload resource. Icky. It would also be okay to say the Location of the upload resource MUST NOT change if Location is sent more than once. A more robust approach would be a request for creation of a temporary resource with a 0-length request body, which is separate from requests sending the request body to the upload resource.

This sounds like it would be design change, albeit a minor one.