`Make sure the vent is opened.
have a plastic pin-like (~1/8" dia) that plugs a small hole on the face plate of the fill fitting. Like - next to the fill pipe opening. Usually the pin
is part of the fill cap retainer that keeps the cap attached to the face plate while filling.
Check it out, it may be that simple (if you're lucky
If the vent is not breathing, a high volume of water will back up into the fill hose. Further, even with the vent breathing (quite a small ID) if you fill too fast you will also experience this condition.
Maybe you just need to fill half-fast!