Just tested it. A Lemmy-Post is only created in one community, even if other communities are mentioned.

An external post (in this case from WordPress with the Activity Pub plugin) is only posted into the Lemmy community that is mentioned first.

Meh, I don't see this as a problem. People already crosspost into multiple relevant communities. If anything doing so with one post is a feature that's missing from Lemmy/Piefed. Or is it missing? I'll try posting into multiple communities after this comment.

It seems to me that your problem is more aesthetic in nature. Maybe ask your favourite UI to make a feature to hide mentions and hashtags that are alone on their line.