I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “!communityname@instance.domain”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
I have in fact noticed a 1-2 minute delay when searching for a completely fresh comunity from lemm.ee. Initially the search turns up 0 results, but trying again in a minute will find the community.