To a 98% approximation, flight attendants don’t actually care if you use the lavatory while the seat belt sign is on. They have to tell you that the seat belt sign is on. They cannot tell you it is ok for you to use the lavatory.
I watch passengers, over and over, asking permission. The flight attendant cannot give you permission. Because what if something bad happened? That’s on them and the airline. But if they advise you that the seat belt sign is on and you go anyway it’s pretty much on you.
Comedy Central’s Key & Peele take on using the lavatory while the seat belt sign is on…