Always finish both sides of a piece the same way.

You want the movement of water vapor to be equal on both surfaces to prevent warping and cracking, espcially important when you have glued aup a large surface.

It seems from your description that you have made what are called "breadboard ends" cross-grain on the tabletop. If these flush battens are not installed to allow for wood movement, you will get cracks. Typical mistake is to use tight mortise & tenon joints. Actually, the tenons have to be smaller than their mortises to allow for shrinkage. The center m&t can be tight so the batten can't wander.
Casey

Actually, if all the measurements he gave are absolutely correct, the two 4" by 36" pieces would have been butt joint glued---no mortise and tenon. Maybe using a spline or biscuits?

No matter, either way it sounds as if wood movement was not planned for or designed in.

Plus, the advice for using sanding sealer-----the name should raise a red flag------as a finish was really bad.

To the OP, wood moves. Period. there is little that can stop all movement. Things like alternating grain when gluing long pieces help minimize movement. Allowing space in the mortise of a breadboard end so the tenons can move is another. And then pinning the breadboard ends in place instead of gluing solidly.

Finishing is designed to have multiple uses. Beauty, protection from harm, and moisture control are the main three uses. Controlling moisture from seasonal humidity changes helps prevent unwanted movement. But, the finish has to cover all the surfaces of the wood. Or, there will be more moisture sucked into the unfinished areas. That actually intensifies wood movement problems since half the wood is protected and half is not.

You need to remove the sanding sealer and use whatever finish you used for the top side all over.

If you are lucky, you used a lacquer based sealer---lacquer thinner will take most of the sealer off in that case. If it is oil or water based sealer, you will need a stripper or just try and sand most of it off. Mahogany is porous enough there will be some left in the grain.

That is probably the best you can now do.

I agree completely w/Casey and Handymac; this sounds very much like a case of faulty construction, not waiting too long to finish. Assuming you've built it the way we think you built it, stopping the cracking altogether means rebuilding the top.