If an instance admin stickies a post it'll show up at the top of all your feeds (unless you sort by "top").
However, I checked lemm.ee and it doesn't have any stickied posts, but it does have a month old post stuck at the top of "all". Comments in the thread are complaining about it. I see lemm.ee is using the newest version of Lemmy (v0.19.10) so maybe it's a new bug?
The main feed, sorry I thought I put that part in the title the one that shows stuff from all your communities, I thought the decentralized format here means a sticky like that would be only in its own community?
So while this isn't the cause in your case, it is possible to sticky things to the main feed. It's done by admins, and for example lemmy.ca has a welcome post stickied right now: