@iLikeDirt covered a lot of important information, but I'd like to add something since the majority of the information assumes that there is a problem of excess moisture due to the concrete, while the expert assessment says it is actually dry cupping that is the problem.
Dry cupping implies that the interior humidity is dropping below the average frequently enough that the wood surface dries out. It sounds like they are suggesting that the vapor barrier between the slab and floor is probably doing its job correctly.
One way to help prevent this is to bring the wood into house and allow it to be exposed to the air within the home so the moisture content is similar to that within the home (acclimation). I like to assume this is standard practice and this was already done, but if you're using a special reclaimed antique pine wood, then maybe a longer period would have been desirable. Major seasonal humidity variation can still have an impact.
Do you know if there is a period during the year where it seems more prominent, or has it not been long enough? If I had to guess, it could be during the summer when the AC/dehumidifier is running more frequently. I'm not as familiar with the climate there, but in parts with cold winters you are more likely to encounter dry air issues during the cold months.
One thing that could be happening is that the glue on the underside of the wood is acting as a seal on just that side, inhibiting the wood from 'breathing' underneath. The only surface area for moisture to leave the wood is on top. A solution that allows the wood to better breath on the bottom, or simply avoids using glue or too much glue, might help. But then you need make sure the vapor barrier is doing its job or you will end up back where you started with too much moisture underneath, the expert seems to imply you no longer have that issue though.
You can still end up with this problem with engineered floors. It could still be a problem with an unknown source of moisture throwing things out of balance as well. I'd see what the neighbors are doing as a solution for your area and if they have similar problems before I spend more money.
it this case, it is not a home improvement project. Since it is a commercial building, you really need to have an engineer design a fix. the liability is to great in rental or commercial property not to have it endorsed. Don't take a cheap fix, it will bite you. The safety of a lot of people depend on how you fix the problem.
Best Answer
i would start by digging out the area a little bit to see how bad the hole is. if its not to deep or just a sign of a bigger problem, i would fix it like so:
1) using lacquer sticks and an alcohol lamp (you can get these at lee valley or rockler, etc) melt down different colours until you get approximately the right colour.
2) get a little crucible, like a small soup ladle or something. heat up twice as much as you think you will need to fill the void. once its all melted, pour into the hole (you can patch any little cracks or holes internally with kleenex, plasticine, whatever - prior to pouring). the heat and the chemistry of the lacquer sticks will let it flow into every little crevice and bond tightly to the existing wood (like a dental filling). fill until its just at the surface.
3) once the lacquer has cooled, it will suck in a little. now put more lacquer in as necessary using burn-in method (just check youtube for how to do this - its an art, so be patient). you may have to use three or four colours to get the grain to match perfectly.
4) once done, scrape flat with a razor blade and buff. now you will have a perfectly invisible, touch patch that wont pop out or disintegrate in time.