I think there's an issue with the fares DL has filed for FWA-HND for RT travel. OW fares have no issue. However RT it appears there's an error with filed fares. I can't even pull up the actual details of some of them, it gives an error.

It's definitely an issue with fares and restrictions on date of week travel. It appears something has been misconfigured. It may be DL IT misunderstanding the date restrictions in fare or bad info in the fare. Since DL has been updating reservation systems, my guess is DL IT. Ie if you change to Sunday departure and Tuesday return, no issue booking on DL app.