This might be worth pursuing through a support ticket as one of the things I'd likely want to collect/investigate would be a Clarity SQL Trace of the XOG user performing the requests (both the write and the subsequent read), which may reveal the clues as to why the attribute and values are missing or excluded in this occurrence.
It would be best also if you could have your process/GEL generate the XOG write file to disk (with <gel:serialize> or similar) so that this can be a repeatable exercise whilst the testing is conducted. Make sure to use the same XOG user outside of the GEL/process for the setting up of the Clarity SQL Trace and performing the standalone tests for consistency (and if your previous XOG read used as the basis of the definition of the write files was performed as another XOG user, it might be worth repeating the same tests with that user also).