By default, a piefed/Lemmy instance only knows of the existence of its own local communities. To see any off-instance communities in the search, they first have to initiate federation by a local user manually searching that community and/or subscribing to it.

Once the off-instance community has been federated by a single user, it will stay that way forever, and other users on your local instance will see it show up in the search as well.

There is an effort to automate that initial federation by Lemmy-federate, which creates a bot on participating instances to automatically subscribe to participating communities, but I’m not sure if piefed instances are compatible with it yet.