That sounds like it could be a plausible answer, so I looked for some posts called “Title” (or some variation), but both the users I happened to find who did this have a Lemmy site as their home instance.
So there must be another reason. I also wasn’t aware that your posts from Mastodon could federate to Lemmy. I thought the two platforms were incompatible, since here all content has to fit into a specific community, but there you have to use hashtags, which don’t seem to correlate to any specific Lemmy communites (though I’ve only lurked on a few Mastodon instances; I don’t have an account)