[rohrmann]

It's the check valve in the water pump that is allowing water to flow into the fresh tank. Depending on which pump you have, the check valve may hang on the outside or it may be internal, but in either case, there may be debris causing it to not seal or the seal has failed. I had that problem years ago and fixed it by installing an inline check valve in the PEX on the pressure side of the pump. In the photo, it is the brass device in the lower center.