Because of the forces involved, the rotor side will want to slip backwards when braking, the drivetrain side will want to slip forward, as you found out.
The dropouts of that model are wrong. Both screws are at the back, therefore nothing stops the forward slippage of the drive side. I would fit the screws and run it as close to the front as possible to limit that. I run mine the opposite to how you do, all the way to the front.
Ah nice thanks for the blog post link. On mine the drive-side screw does go in from the front, so my Straggler must be the first version where they made that change. I've now fitted the screws (with the wheel in the same position as before) so hopefully extra protection against slipping.
Because of the forces involved, the rotor side will want to slip backwards when braking, the drivetrain side will want to slip forward, as you found out.
The dropouts of that model are wrong. Both screws are at the back, therefore nothing stops the forward slippage of the drive side. I would fit the screws and run it as close to the front as possible to limit that. I run mine the opposite to how you do, all the way to the front.
They quietly fixed it in the following year models.