This is the reason:
Looks like the PieFed’s old post deletion does not take into account whether a poster is local to a deleting instance or external to it.
At least one banger of mine on this comm is already affected.
Memes on piefed.zip should be safe.
Some PieFed stuff, like this, that or that has been already deleted from PieFed. Discussion and links survive on a comm instance but images don’t (Lemmy does not cache images).
Filed a bug about it.
None of those are from a PieFed.zip account. The origination source PieFed.social that you sent the meme from is what deletes the content, not the destination https://feddit.uk/c/fedimemes nor the 3rd party bystander PieFed.zip.
In the community sidebar as viewed from PieFed.social - which also shows below every single post - there is a notice that “Posts older than 365 days will be removed.” So not so much an unintentional bug as poor design to inform users about deletion of content. The documentation could be improved to avoid disappointing people in the future.