I'm with them and don't forget the washing machine drain has a trap in it. The top of the pipe is around the back and the top of the machine but the trap is just around floor level.
I'd also try the traps first but partially plugged or insufficient vents will allow flushing/flowing water to suck nearby traps dry. It helps with some traps that aren't used much, such as floor drains, to add some vegetable oil for a top layer so it doesn't evaporate.
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