This was originally reported as a documentation issue: #357 However, I'm experiencing it while using 3.0.1.
I believe this is because the __all__ special property omits some of the flags specified later on, including SENT. When I fiddle with the source locally, adding "SENT" to __all__, this works fine.
https://github.com/mjs/imapclient/blob/3.0.1/imapclient/imapclient.py#L36-L45