They call it morning sickness.

usally caused by pump bushing clearance. The converter leaks down while it sits unused.

Since a TF doesn't have line pressure in park, you are waiting for the converter to fill up when you shift it into gear.

As suggested earlier, it should improve if you warm the car up in neutral, instead of park.

I would bring it up with the rebuilder, but if it works normally after warming it up in neutral, I'm not sure I'd worry about it much.