had this same bug happen to me too around the same time after the plate dropped, was unsure if it was meant to stay for a while or not
Yeah, it happened to me after I washed the first plate and picked up the second one. When I turned around with the second plate in my hands, a monster appeared, the screen blacked out and when the game came back the plate was just gone. I figured it was supposed to be on the ground, as in the character got startled by the monster and dropped it, but instead of being on the floor the plate was just gone.