does the water drain OUT as fast as it should or has been?
Have you ran a slow manual careful snake down that drain recently?
Are their other appliances in that room that could be the culprit?
May be gross, but have you actually got down on your hands and knees and smelt that drain? I am wondering if you might have had an animal to die in the walls around that room and it just now has started to smell more.
I am fishing for you as you may realize. These things are difficult especially since you stated that the p is draining and holding water.