The `pallet_bridge_grandpa` at Polkadot Bridge Hub is now in an inconsistent state, the Polkadot-GRANDPA-light-client running BH doesn’t correctly handle a corner case involving validator set changes following validator slashing. As a result of the bug, the GRANDPA light-client can fail to recognise the other side's validator set change and thus start refusing to accept subsequent validator set changes as well. More details can be found [on this forum post](https://forum.polkadot.network/t/polkadot-kusama-bridge/2971/45) .

We have encountered this bug on the BH, resulting in Kusama->Polkadot traffic being stuck. This upgrade fixes the bug so that it is protected from encountering the same issue.

Please note that there is a Polkadot runtime [upgrade](https://polkadot.polkassembly.io/referenda/648) (to 1.2.0) already up for a vote, so the goal is to wait for that to happen before enacting a subsequent patch fix on top.

Also, be aware that this proposal is submitted on the Whitelisted Caller track - a parallel submission should be up for a vote on Fellowship to whitelist this proposal shortly.

See [`Pallet::dispatch_whitelisted_call_with_preimage`].

- **Problem Statement:** `pallet_bridge_grandpa` at Polkadot Bridge Hub is in an inconsistent state, affecting Kusama->Polkadot traffic due to a bug in the Polkadot-GRANDPA-light-client.
- **Significance:** Bug causes failure to recognize validator set changes, leading to traffic being stuck.
- **Proposed Solution:** Upgrade to fix bug and prevent future occurrences.
- **Expected Outcomes:** Resolution of bug, ensuring smooth traffic flow between Kusama and Polkadot.

#694 Polkadot Bridge Hub 1.2.2 upgrade: Unstuck bridge with Kusama