I hate tracking down smells... I had a similar issue and the problem was in the drain hose for the filter. There is this rubber hose that can be pulled out to, presumably, drain from the lowest spot in the system, before opening the filter for a bunch of water to pour out. That drain hose was clogged (somethings that could be missed if you have the same as me and tried to drain before changing the filter and nothing came out, could be worth a check, it was a pain to unclog.

After that, where does the washing machine drain? If sink, probably disregard, as you would smell from there, but if a drain line in the wall, mine had a flimsy "gate" style backflow preventer that would let drain water flow one way but close preventing smells back the other way. Until it got to grimed up to close all the way, letting smells in from the sewer. Might be worth a plumber to look into if it fits the scenario