This is wildly out there, but what if there was/is a hitman, and FD was supposed to stay away for his alibi.
However, things went sideways, JD mentions to hitman that FD doesn’t have the money to pay him, and hitman plants FD DNA evidence to stick it to FD? Also, this leaves FD with the cleanup. Or hitman cleans up but sloppily because no paycheck, so he just makes sure his DNA isn’t there. FD hears JD is gone, thinks everything is going according to plan and is excited. He doesn’t realize the poop has hit the fan.
Hitman may have carried out the job anyway, (perhaps JD recognizes them so they need to for their own safety?) but this would explain FD’s statement about not leaving any DNA in the house. If FD knew the plan, then he 100% would not want any of his DNA in the house and this might explain why the scene and timeline is weird.
Completely my own conjecture here.