The problem is probably coming from where the undermount of the sink meets with the wood. I’m guessing there’s probably a good chance your finish may have been damaged during installation. It’s pretty easy to damage a couple layers of PU.

From my experience with PU, you prob want more like 4-5 layers instead of two, and I would suggest sealing the seam where the sink meets the wood with some silicone.

As far as the damage goes, you’re prob going to want to put in a dove tail and refinish it, or you can try impregnating it with epoxy if you have any experience with it.

Tbh this just isn’t a good place for butchers block, too many end grains exposed from a variety of wood that swells to different degrees. Ya mix that with heat and humidity and you got yourself a bunch of different places problems can pop up.