Mastodon posts tend to get funky when they federate, because Mastodon has this (annoying) norm of starting posts with mentions. So OP mentioned the community on the first line, which became the first part of the title - @fediverse. Second, it mentions Kagi as a tag instead of name, which gives the @kagihq. And then comes the first sentence as the rest of the title.
I'm not writing from Mastodon, but from Friendica
It's a great example of Lemmy/Mastodon interoperability working, but not being quite there yet.
The mentions problem seems to be only for PieFed, but it doesn't happen with Lemmy
I think it should be reported as an issue to PieFed developers