I am seeing that this is the same bug listed on issue REALMS-9150 here on the bug tracker.
I am having this same issue, however, I am on Windows 10 not Xbox Clubs, but it is the same issue.
Click on Esc, then Camera Icon to take a screenshot, gives the status that the Screenshot is being uploaded, then another saying it was uploaded successfully. When going back into the feed log, it does not show the posting of the screenshot I had just taken.
The last successful post that is one the Realm Feed that I am part of is from March 8, 2021.
Hello, This is still an issue as of 1.18.2. The image that has already been submitted is the same message I am getting.