If the unit will do this more on the rinse cycle it will likely be the the drain hose is too far down the drain pipe or the end of the drain hose is below the bottom of the tub. Just raising the drain hose to the correct height will fix this issue. Check with item #10 in the look up below to make sure you comply with the installation.
Click here for install manual
Thank you and please let me know what you find.
Is the water going out of the drain hose when it is not filling all the way?