My outhouse gets full, i force a dupe to clean it, they do their plunging and then run off with the muck to dump it into the compost. At this point when they leave if i hover over the outhouse it says 15 uses remaining, but the little arrow graphic on the icon still says full. That graphic doesn't get reset until another dupe uses the outhouse next.
screenshot attached, i have the outhouse in question selected and they dupe is dumping at the compost pile on the far left.
fill outhouse, force to clean, when dupe runs to compost pile with waste note the graphics/info