Lavatory seems to be "stuck" or something, someone uses it, but then it doesn't become available again even though it has water and everything. It says lavatory in use, but there's no one there.
Also, sometimes it just doesn't show any status (ie ready, in use, etc) and duplicants won't use it, they will just make a mess.
Had an outhouse, built a lavatory. At first it said lavatory ready, and then after using it once it was stuck at no state (not ready and not in use). Deconstructed and reconstructed, the issue was fixed, but later on it became stuck as "in use" although no one was using it.