An attachment link can depend on its retention, current authorization, and the supported media retrieval path. A message reference is not a permanent public download address. An expired private link does not prove that the whole message failed.
Use the authorized retrieval action again if it is available, and inspect the returned error. Do not make the file public or share a signed link to work around access. If media from an eligible retained period remains unavailable, report the message reference and time to support without attaching unrelated customer files.