fix: resort UE_list after slice multiplexing

- slice multiplexing resorts UE_list for its purposes with restoring the
  original order
- in order to keep the order in UE_list, resort to original order after
  multiplexing
- ideally, this would happen in the multiplexing but currently, the slice_idx
  is not passed to multiplexing (it does not care), so this cannot be done ATM