Data race condition in ‘boost/thread/lock_types.hpp’

Hi,
I've just run Coverity, version 2020.03, static analysis on a program using Boost v 1_75_0
It has highlighted several potential issues one of which I will describe here.

In lock_types.hpp the ‘unique_lock’ class contains a mutex and a Boolean guard, called ‘is_locked’, is checked, via the ‘owns_lock()’ method, prior to locking and unlocking the mutex and then set to true or false respectively. For example, on line 451 the mutex is unlocked and on line 452 the ‘is_locked’ is set to false. Another thread can at the same time call owns_lock() to check the locked status and potentially get wrong answer.
This, to me and the Coverity static analysis, creates a data race condition. Is that a false positive in the static analysis tool? What am I missing?

Thanks,
Dimitar