I have a couple of top-level large assemblies pinned to my Recent Documents list.
When I went to open one, and another user already has it open, it offers me to open a copy. I decline this.
Then, that file disappears from my recent files, either pinned or normal.
Edit: One file is properly opened, it reappears pinned in recent list, without needing re-pinned.
(I can re-pin it and) I repeated the issue under same conditions.
Ref. 2018 SP2 installation, updated Windows 7 Pro x64 OS