This may not be as wide a bug as stated.
My lemmy.ca account (and the two lemmy.ca communities that I follow via Mastodon) has still been federating, and they upgraded Lemmy to 0.18.0 a couple days back.
Searching for lemmy.ca also brings up other communities in my search box.
Posts I made there earlier this AM were visible instantly here.
@nutomic well @srgray, this issue isn't actually about federation persay, its about something changing with how webfinger is handled. if you want to test with your instance, try copying the link to an individual comment from the fediverse icon on lemmy and searching for it from mastodon. that used to work just great for every instance before 0.18 and no longer works for me and many others.
the admin of lemmy.ca has been trying to manually fix this issue so if it does work for you thats why