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.

https://www.instructables.com/Inlay-a-Bow-Tie-by-Hand/

Makes a wooden joint that holds the pressures in the wood that are causing tge seperation.

Maybe something like: https://www.homedepot.com/p/Everbilt-1-1-4-in-x-5-ft-18-Gauge-Thick-Zinc-Plated-Slotted-Steel-Angle-15152/332733420

Not aluminum, but galvanized steel, so should be ok in moderately wet environment. Plus, it’s cheaper than the 1/8” aluminum angle at HD. You could prob get 2 strips from each, or they may even have longer lengths for cheaper.

You’d just have to make sure you’ve got 1.25” clearance (e.g. don’t screw it in right above a drawer).

Of course, one of the things with butcher block is that it’s gonna expand and contract with humidity, so you can’t completely restrict it. But I think that if you use screws that have diameters that are smaller than the pre-drilled holes on that angle, even when they’re screwed in tightly, they’ll have adequate lateral movement.

Disclaimer - I’m not a “carpenter” by any means, but I do nearly all of my own home repairs, and the above is what I’d prob try if faced with your problem.

regularly should I oil the and wax the block? I have been applying conditioner regularly, but it didnt feel like it was doing anything. the board has been treated with several layers of polyurethane.

Never, it's not going to do anything as it's not going to permeate through the layers of poly.

You may be getting confused with the butcher's block used for food prep and as a cutting board which you don't usually leave unsealed, or sealed with wax.

Also, a sharp chisel and patience is all you really need for bow ties. Try some practice runs on some scrap and see how you fare.

With a poly coating on there it’s more difficult to say.

Personally I keep an eye on the colour of the wood. When it starts to look dry and pale, it’s time.

The wood will let you know, it will stop soaking the oil and wax and you then wipe off the excess and start buffing.

It may let oil seep out for a few days after, as it adjusts. Just wipe that away. Food grade mineral oil (which I use for all my boards) is non toxic, but it is a laxative, so do wipe up and excess before using the board 😁