EDIT: I’d been 11 hours since I posted this. Scott Frakes isn’t a real person. Jonathan doesn’t have a twin brother. His middle name is Scott. This is a long running joke from Mission Log podcast. Whenever Thomas Riker was in an episode or they mentioned him, they would pretend he was played by a person named Scott Frakes, the imaginary twin brother of Jonathan Frakes.
It’s an honest shock that he came back in a different series.
But yeah, they should have finally gotten around to rescuing him during the Dominion War or something.