This usually happens because the water is being siphoned out of the drain hose while it is filling.

This will happen if the drain hose is pushed down into the drain pipe below the tub level, or if the drain pipe itself is below the tub level or if you have taped the drain hose to the drain pipe and shut off the air to the drain pipe, creating the siphon.

Check it out, and let us know what you find. I am guessing one of these is your problem and it is an easy fix. If not, post what you found along with your full model number.