not sure why this happens, but make sure you check out the phys.body.velocity example patch that demonstrates how to query and control the velocity of a rigid-body:
you probably want to make sure you are upgraded to 6.0.8 too.
Controlling velocity is a good workaround but maybe it is better fix the problem around restitution
I searched in the Bullet forum and i found this thread http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=7922&hilit=restitution
the administrator speak about:
"enable split impulse, it will prevent the added energy (velocity) from penetrations.
m_dynamicsWorld->getSolverInfo().m_splitImpulse = true;"
Is possible to control split impulse in jitter?