As I explained nearly three years ago generally 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.
Now, of course, you must follow flight attendant instructions. So if they actually do tell you to sit down, you’d best do it. Most of the time passengers think a flight attendant is telling them to sit down when they’re just saying the seat belt sign is on. Occasionally I see flight attendants saying ‘the seat belt sign is on’ while making faces and motioning passengers into the lav.