Did you make sure the stopper is centre over the hole in the sink?
If you just tightened it up it might be to one side of the hole. A bit of insulating tape on 4 sides an even margin away from hole would help to show it correctly centred.
I seal those stoppers with some clear silicone or Tec7 also.
In fact, the rubber washer just needs below the sink if you use silicone above.
To seal with clear silicone, do you only need a small smear under the stopper itself - and the black rubber washer put under the sink next to the plastic screw?
I have some old LS-X from a toilet repair - would this work the same as clear silicone?