Calum -
My SS test set up is currently unavailable so I can't see 1st hand what happens with a SS right now.
However, on a PP, the actual feed rate DRO = F word * FRO % to within +- 1 of the least significant digit.
Also the FRO does change the FR as the FRO is changed.
So, if there is a bug, it would appear to be SS/Mach related.
Dave