In response to SP's request, my own experience is as follows.
When I post a meme, it's invariably sourced from FB and can be viewed at the time of posting and thereafter
When I post a link from YT, this can be viewed initially, but, subsequently vanishes when I leave the thread. Equally, other posters attaching clips from YT cannot be viewed either. Others have suggested clicking on "reply " and the link will appear, and, whilst this does happen, clicking on the link to open doesn't work.
Clicking on links contained in threads to other sources, invariably does work however hence the issue seems to be YT related.
Hope this helps with the fault finding.