Leaks from a drain under a sink are maddening. I had one in the guest bath and replaced the nylon washer, still leaked. After a few years I had a plumber in the house for a different issue which he fixed (can't remember what it was), and he asked if there was anything else I needed him to look at. I had him check the drain in the bathroom, and it was a tiny little crack in the pipe coming out of the trap connecting to the pipe going into the wall. The crack was actually inside the pipe that enters the wall. Just enough to cause a drip.

Good luck getting the leak resolved. You've worked so hard on that bathroom.