The `flush()` method unconditionally sets `self._active.flag = True` and resets it to `False` in the finally block, without first checking if the flag was already set. If `flush()` is called while `_flush_loop()` is running on the same thread (which permanently sets `_active.flag = True`), the finally block will incorrectly reset the flag to `False`, defeating the deadlock protection. This could allow a subsequent `add()` call to proceed and attempt to acquire locks that are already held.

When `_ensure_thread()` returns False or `_flusher is None` at lines 65-66, the method returns without clearing `self._active.flag`. Since the flag is set at line 63 **before** the `try` block that contains the `finally` to reset it, this early return path leaves the flag permanently True for the current thread. All subsequent `add()` calls from that thread will be silently discarded. The base class `_batcher.py` correctly places this check **inside** the `try` block (lines 94-96), ensuring the `finally` always runs.