This is a classic question I get at least once a year, regardless of TFS, VSTS, versions, etc.
I cannot see my Product Backlog Item on the board anymore! Is it gone? Are we losing data?
No, we are not losing data, but you need to remember that if you are linking a PBI/User Story as a child to another one the parent won’t be shown in the board, and that is by design.
This is something that dates back a few year – Willy covered it in 2013 but it is an older discussion theme, always worth remembering it though