A state consistency bug in x/costaking can leave a BTC delegator with non-zero ActiveSatoshis (Phatom Stake) even after they have fully unbonded their BTC delegation, if their Finality Provider (FP) drops out of the active set in the exact same babylon block height. This creates a “phantom stake”: the delegator’s BTC capital is withdrawn, the FP is inactive, but costaking continues to treat the delegation as active BTC stake allowing ongoing rewards accrual without backing BTC.

An address can keep earning costaking rewards with zero BTC staked.

Reported by @BottyBott.