Instead of the mStable MTA/WETH Staking BPT (mBPT) being held in the Staking Token BPT (stkBPT) contract, it is deposited into the new Balancer mBPT Gauge Deposit (mBPT-gauge).
MCCP 21: Stake BPT staked in mStable into Balancer Staking contract has details on why the change was done.
See Pull Request #350 for the detailed contract changes.
Full Changelog: v2.4.2...v2.4.3