I've seen photos of a Kittiwake 23 where the deck beam beneath the mast had deformed, cracked and bent considerably beneath the mast step, but sadly the photos aren't on-line any longer. I do have a pdf of the photos that I would email to you if you wish.
Note, as the cabin top sags, the shrouds slacken. Re-tensioning the shrouds increases the stress on the cabin top, causing it to sag more. Continuation of the process will crack the underside of the cabin top beneath the mast step as you convert from a deck stepped mast to a keel stepped mast.
This what I believe happened in the Kittiwake photos.
There are some threads here
that discuss this issue.
A deck stepped mast needs a beam or bulkhead under the step to carry the loads to the hull sides and or through compression post(s) down to the keel structure. It sounds as though your boat doesn't have a visible
beam, or maybe not one at all, but I think that's unlikely. Possibly the post(s)was moved?
The need is to provide, beneath the mast step, proper structural support that will carry the mast compression loading down to the keel structure. This is usually done using a bulkhead (which can also resist and balance out somewhat mast loads and chain plate loads from the shrouds) or a compression post (or two). If the compression post on your boat is offset from the mast, you need to figure out how it was intended that the load be carried down... A post offset fore or aft is structurally inefficient, it results in eccentric loading, lots of bending of the post and also needs a very strong cabin top to resist bending there. Was the post ever relocated?
Usually, directly under
the mast step, there's a bulkhead, or a beam under the cabin roof at the mast step with compression posts either side of the passageway opening (see how it's done on the Kittiwake for example)
After you determine if the post was ever moved, check inside the cabin top under the mast step for cracking or other damage. A bulkhead, a post, or beam and posts under the mast step
would most likely solve your problem and return structural integrity to your boat's hull. Such a beam can be composite aluminium and wood or fg, cambered to match the original camber of the cabin roof, and well tabbed into the roof, and properly supported by posts or bulkhead to the keel. Not too too terribly difficult (mast off) but you might need a beam or bracket at keel level also to pick up the loads at the bottom...
Good luck, let us know what you find and what you do!