I would check with SE16 what the last change date was for your company code in table MARV.
if this change date was after the entry date of the material movement then it can be assumed that the back posting was not forbidden at that time.
Despite of the fact that certain activities are still possible, a standard cancellation movement is not possible. Details can be found in 2014728 - Back posting of Service Entry Sheet or Goods Receipt and 1134322 - MMRV and return postings to the previous period