It didn't took into account the fact that we can get a new lease after changing a TTL. In this case we have to update the exiting leader key with the new lease.
To solve it we introduce the on transaction 'failure' callback. The whole workflow looks like (schematically):
```python
txn(
compare=(old_value == self._name)),
success=put(self.leader_path, self._name, self._lease),
failure=txn(
compare=(create_revision == '0'),
success=put(self.leader_path, self._name, self._lease)
)
)
```
The problem was introduced in d98d6d0b02c9cc67464a7b1b31b1c5570c26e12d