[Flyinfool]

Try it in a vertical nose dive also. It will do the same thing.
It should roll right with a clockwise rotating head.
This is due to you having a small amount of right aileron trim to counter the tail thrust, which is countering the main rotor torque. If you notice the heli will always lean left while in a hover.
There is a balancing act going on where the main rotor must counter the thrust of the tail rotor.

I trim my heli for a neutral setting where it will drift left in an upright hover and drift left in an inverted hover to reduce the roll coupling at speed.