This is early for the neutral drain mechanism to fail but it's certainly not unheard of. The actual neutral drain cam is a relatively inexpensive part, but it is quite a project to replace it. I haven't attempted it yet. (I do have a couple of WP DD trans. that need this repair in the basement).
Most service techs just drop the offending transmission out and replace it with either a new or rebuilt one. If it has been doing this for any period of time you really should also replace the spin clutch at the same time since it is most likely very worn from spinning with a full tub of water. And, since you have to drop the tranny out to replace the cluth, it is the sensible time to do this.
Now, you also have consider if you want to spend the money to fix this. A new DD tranny is usually in the $180-200 range, a rebuilt is $90-120. I think I paid about $50 for the last new WP DD clutch.