Record the pair, not just the fragment
A fragmented MP4 delivery package has media and an initialisation context. The W3C ISO BMFF byte-stream format note describes their separate roles for Media Source Extensions. A successful fragment download does not establish that the decoder received the matching configuration.
For an invented release, generation A and generation B both use init.mp4, while B changes the encoded configuration. If an intermediary still holds A's initialisation bytes, a client can receive a mixture that no release reviewer intended. This is a possible production explanation, not proof of any particular playback error.
Gather generation evidence
Record the manifest revision, initialisation resource identity, validator and media fragment generation. Compare a response obtained through the normal delivery path with the release artifact. A filename match is weaker evidence than a byte identity or a release-controlled digest.
Include cache headers and response age where available. A fresh origin fetch cannot by itself establish what an earlier client received through another cache. Keep those observations separate in the incident timeline rather than declaring the fault resolved from one request.
Publish a coherent generation
One production approach is to give changed initialisation bytes a new stable URL and publish references to the completed generation together. That is a design choice to validate against the delivery system, not an assertion that every format requires a particular naming scheme.
Retain the old generation long enough for the intended handoff and active readers. Avoid replacing a configuration file in place simply because only its label appears in a manifest. The acceptance question is whether each advertised media path resolves to its intended configuration throughout the transition.
Questions answered
Does a 200 response prove the initialisation file is correct?
It proves a response was delivered, not that its bytes belong to the advertised media generation.
Is this W3C note a universal decoder-support list?
No. Its scope is the ISO BMFF byte-stream format for Media Source Extensions.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.