I have the same issue starting today. There used to be 2 modes for filling in the password fields. Standard and NEW (Experimental). The New came on as default a couple of years ago and very few of my sites worked anymore, so support told me to turn it off.
Today, I recognised that the interface had changed AND password are not being filled in, BUT, they have removed the option to go back to OLD way, so we are stuffed and I am going to leave LP as this is a BIG PAIN for me.