I don't know about Jerboa, but this happens on the web too, and it's usually on more fringe / less popular communities. There's a workaround that works for me, I mostly heard about it from someone else here on Lemmy. It's not great user experience, but it works.

• Go to the search function;
• Type in the name of the community, hit enter;
• Select "Commnities" on the search type dropdown (yes, you have to do it after submitting the search as per the previous steps, seems like another UI bug);
• If after doing the above the community is not found, then here enters the magic sauce:
  • Change the search terms so the community is fully qualified / keyed as [!communityname@instance.server](/c/communityname@instance.server), all in lowercase. So, for example, if you're looking for the "kreisvegs" community on feddit.de, type [!kreisvegs@feddit.de](/c/kreisvegs@feddit.de) on the search field;
  • Submit. It still won't be found. Give it a few seconds, change back the search terms to only the community name (without the instace and "!" at the start). Submit. It should now show up.

You may have to repeat step 4 onwards a couple times, sometimes.

My uneducated guess as to what's happening: the federated communities listing is probably cached on the instance, and by default it'll only look for communities cached on your instance. My guess is that federated communities only gets into the instance cache when members of the current instance have searched / subscribed to that community. Typing the fully qualified community name on the search field (which is the tip I got from someone else) apparently forces the search function to actually contact the external instance to look for the community, instead of looking in the cache, but that can take some time, hence why you should wait a few seconds on the 6th step. That guess could also explain the problem also happening on Jerboa, since the problem would be server-side.