Nobody there had subscribed to those communities yet when you did the search.

Search, wait 5 minutes, and if the server didn’t pull the data yet, do it again. Fedi is a bit slow at times, the first time it learns of new communities.

In my experience, you just have to press the search button a few times with a short pause between to let the server fetch the data from the review instance. Or just reload the search page after pressing search.

The lemmy.world instance had been having trouble, could be a problem on their side.

Don’t know if this is related, but I’ve found when viewing posts from “All” communities it really doesn’t seem to show much beyond my subscribed communities. But other more popular instances seem to show a much more varied “All” page. Is this just because this instance doesn’t know about all those various communities?