Fixup backing group computation time (#3170)

Fixes: https://github.com/paritytech/polkadot-sdk/issues/3165

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

polkadot/runtime/parachains/src/runtime_api_impl/v7.rs

@@ -45,6 +45,8 @@ pub fn validators<T: initializer::Config>() -> Vec<ValidatorId> {
 /// Implementation for the `validator_groups` function of the runtime API.
 pub fn validator_groups<T: initializer::Config>(
 ) -> (Vec<Vec<ValidatorIndex>>, GroupRotationInfo<BlockNumberFor<T>>) {
+	// This formula needs to be the same as the one we use
+	// when populating group_responsible in `availability_cores`
 	let now = <frame_system::Pallet<T>>::block_number() + One::one();
 
 	let groups = <scheduler::Pallet<T>>::validator_groups();
@@ -95,6 +97,11 @@ pub fn availability_cores<T: initializer::Config>() -> Vec<CoreState<T::Hash, Bl
 						.expect("Occupied core always has pending availability; qed");
 
 				let backed_in_number = *pending_availability.backed_in_number();
+
+				// Use the same block number for determining the responsible group as what the
+				// backing subsystem would use when it calls validator_groups api.
+				let backing_group_allocation_time =
+					pending_availability.relay_parent_number() + One::one();
 				CoreState::Occupied(OccupiedCore {
 					next_up_on_available: <scheduler::Pallet<T>>::next_up_on_available(CoreIndex(
 						i as u32,
@@ -106,7 +113,7 @@ pub fn availability_cores<T: initializer::Config>() -> Vec<CoreState<T::Hash, Bl
 					)),
 					availability: pending_availability.availability_votes().clone(),
 					group_responsible: group_responsible_for(
-						backed_in_number,
+						backing_group_allocation_time,
 						pending_availability.core_occupied(),
 					),
 					candidate_hash: pending_availability.candidate_hash(),