A few of us have shared your pain exact issue. I have the water manifold the only shared water line is toilet an cold washing machine.
I think lowering the anti siphon loop a few inches on washer discharge line might keep it from over flowing but not over filling. I bumped the agitator with switch drum moved 1/4 turn just that slight slosh and it slowly siphoned most of out. Maybe if perfectly level it may have started to siphon.
I haven't test this theory but have solved much harder complex design flaws.
95 Monaco Crown Royale
M11 400hp, 4060 trans.
Aquahot, Generac Guardian7.5k