This is a serious problem. I used to be able to report problems. Some were never answered or resolved, but I at least had the chance to report the problem.
Today I noticed the memories' view seemed to be missing many of the memories I had loaded in the past. I tried to re-load a memory and received an error that it was a duplicate memory. I then tried to go to the select from gallery view which only showed a total of 19 out of possibly hundreds of memories I have loaded over the years. The "duplicate" memory was not there so I could not re-attach it to the person.
2nd problem: When I am at a specific person and upload a document into that person's page, why isn't the document automatically linked to that person?
I have also uploaded thousands of FS Memories.
I have never had a problem with them "disappearing"
(though if you have more than 1,000 in a single album they dont display)
make sure that you don't have a FILTER on though (see the "arrange" option on the screen)
Items uploaded to a family tree profile record under the memories tab would be linked to that person.
Maybe the system was unstable or not working at the exact time you were doing this.
Try it again and see if you still have issues.