DSC';s are applied to the flight in which the eligible SC's were earned, and should (and always have that I am aware of) thus apply to the membership year of that eligible flight.
The LB SC's applying to the year in which redeemed has always been an oddity to me and I do wonder why that is, unless it's a quirk of the implementation since I suppose in a way the loyalty bonus is not specifically tied to an eligible flight per se (even if there is a specific flight credit that triggers one). I always thought the LB SC should apply to the membership year when earned but that obviously is not the case.
Still VERY surprised to see if DSC's really did get applied to a new membership year. Very easy to prove I should think given the DSC enitries in activity log also give the eligible flight date. As always happy to be corrected but I'd love to see proof of this actually happening as suggested.