When computing the roll value coming from the handle bone, the code
was using some strange unexplained math. It probably works fine when
the difference with the 'zero roll' orientation is pure twist, like
is the case when called from mat3_to_vec_roll. However, it appears
to break when significant swing is involved.
The issue is fixed by using the proper Swing+Twist decomposition
utility function that was added in a recent version for drivers.
d1657b406e