Hi Tristan,
Routing is not scheduled. It is the order which used to be scheduled. Check in OPU5 which is customizing setting of scheduling parameter.
Backward scheduling requires the date "Start in the past" to allow start date to be proposed in past days. This is not applicable for forward scheduling. You can do all the adjustment also.
Can you place a screen shot for your setting please?
Regards,
Krishnendu.