Its like putting a lid on a can of liquid. You do have a moisture problem. Could be something as simple as putting a layer of visqueen under your crawlspace. Your getting moisture and until you find the 'problem' your flooring will continue to have the same problem.

3. Exterior trim water table - I don't recall seeing any galvanized flashing over the water table, The metal flashing should be easily detected since it extends over the water table trim. If its not there, ask the builder to correct his mistake. 4. Finished floor over concrete - Here there are specs on the barrier that should go below the hardwood - talks about spaces for expansion, plywood sheating, sleepers (which I just google to see what it is), polystrene, plus insulations, etc... - seems these are all to protect the floor from moisture I don't see anything in this detail that would adequately protect the flooring from moisture. There is no waterproofing or vapor retarder membrane shown on top of the concrete where it belongs and it would be obvious in the detail since the membrane must turn up and be sealed at the perimeter. Styrofoam insulation could only act as a secondary vapor retarder since it is interrupted by the sleepers every 16" o.c. The parenthetical note about sealing the "JOISTS" makes no sense; perhaps it was meant to say: "seal the JOINTS [of the insulation board] with tape or fiberglass mesh and mastic" creating a crude vapor retarder above the sleepers with adhesive joint seals that cover 20% of the floor area. This is not a vapor retarder like the ones used for exterior walls; its intended to stop moisture from the ground and it should be a thick membrane with no joints if at all possible and it should not be above wood sleepers. And why specify no insulation facing; were they afraid that a foil facing might act as a vapor retarder? You will need to open up part of the floor to see what was actually done. Also, the wood sleepers are not specified to be pressure preservative treated which is a serious mistake and could be a code violation but an experienced sub or inspector would have corrected that design error. In general, the details appear be poorly drawn and poorly noted. I doubt there was a design professional involved. Perhaps there is a specification that clarifies some of the cryptic and confusing notes. But the fact that the damage is limited to one room is still puzzling. Perhaps there's more water under that part of the slab because of the walk out door and well.

I'm sorry, I didn't see that, yeah polystyrene is the foam i was talking about. It looks like you already have insulation. Capillary action is dragging water right under that whole floor. That looks like a damp corner outside where you have brick. So dumb I missed the water creeping right under your door threshold. Either you have a bad door pan or the door pan has no place to drain to. Either water is coming down the outside wall and going under the door, or water is pooling up in the brick patio and working it's way in. The solution to fix the patio surface is similar to a shower basin. If the flashing is all good around the door, you might need a little catch basin in the patio outside the door to shed water away from the door.

It sounds like the current updates have brought your house into the range where the value meets the purchase price. You might find that removing the damaged wood and replacing with new and then doing a full sand/refinish over the entire floor MIGHT turn out to be roughly the same as removing all of it and installing new LVP throughout. A quick calculation tells you whether or not a patch + sand and refinish is worth it or if a full removal/replacement with a lower value product is the way to go: $4-$5/sf for a full sand and refinish (no patches or repairs to do). You add in the cost of the wood itself (depends on your product but '87 was good for 3" strip hardwood) + cost of labour to patch the existing wood floor. I'm going to guess the price for the wood is going to around $6/sf (if I'm correct a narrow hardwood is going to be an easy product to source). The material = only for the areas requiring the patch. Labour to remove old and patch in new = $3-4/sf (for that area only). Add in some subfloor work for the water damaged areas = $2/sf for the patched areas. Now the full sand/refinish occurs (which is already established at $4/sf). It sounds like a lot....but wait until you do the math for the LVP. LVP (decent quality = $4/sf) + cost of removal of all flooring products ($2-$3/sf = includes dumping fees) + cost of subfloor preparation ($2/sf includes labour) + cost of install ($2.50/sf). Remember the LVP costs are for the ENTIRE square footage of the home. The hardwood patching costs (new hardwood + new subfloor material + labour) is only for the small area needing new product. The full sand/refinish costs are for the entire wood square footage. A little bit of math and you have your answer.