What a worker lease actually protects
A worker can resume after its lease expires. The write path needs to account for that.
A lease gives a worker a claim for a bounded period. The worker might pause, lose the lease, and resume after its replacement has started.
A fencing token lets the protected resource reject stale writes. Each lease grant gets a higher token. The resource needs to compare the token with the latest one it has accepted and apply that check atomically with the write.