My 2010 Outback does that, and will hold for a couple seconds.
It's not the normal working mode of the hill hold assist system. The Subarus have dual brakes in the rear. The outer and obvious disc/caliper/rotor setup, as well as an inner drum brake. Hill assist and parking brake are electronically controlled, electric-motor drum brakes inside of the rotor.
It's the reason why the rear rotor assembly is so meaty beyond the wheel studs:
Here, you can see the inner drum brakes used by hill assist and for parking brakes:
What you're feeling is the hydraulic disc brakes. Whether it's a different part of hill assist that's not advertised, or simply how the driving brakes work, I don't know. But it's not the hill assist/parking brake assembly (the drum brakes).