Oh alright...
1st of all, you didn't mention your location. Seasonal changes in temperature and more importantly, relative humidity play a huge role in having good results with hardwood flooring.
Another question: is this problem spread uniformly or is it sporadic?

You can have properly kiln dried oak flooring "acclimating" for 2 months in a location that has low humidity for that period... and have poor results 6 months later if the climatic conditions change post installation & finishing.
After all... the FINISHED flooring is sealed on the surface, but the BOTTOM of the material is unsealed & susceptible to moisture absorption. This creates an "imbalance".
ANY bona fide Domestic manufacturer will produce flooring within the industry standards for kiln dried hardwood lumber of 6-8% moisture content.

The fact that your flooring is "cupped" indicates that it is probably absorbing MOISTURE from below. Unless of course, your home is radically HOT, which could bake the moisture out of the tops... but that's unlikely.
Wood will expand in the direction of the water or moisture... similar to what happens if you place a DRY sponge in a puddle of water deep enough to dampen only one side.
Something unforeseen (& perhaps preventable) was overlooked before installation.

Perhaps an additional or added insurance policy such as a "moisture barrier" or even a glue & nail-down installation should have been a consideration.

I know that we, for example, take extra steps to ensure the moisture content is kept more in balance between the TOP & bottom of our prefinished flooring... however this is an "added value" operation that MOST manufacturers or flooring experts are hesitant to employ, because it obviously adds "cost" to the production process.
And we ALL know what the typical response is when you suggest to a customer that they may need to consider "spending a bit more" to insure optimal results.

You'd be wise to consider checking the moisture content in the SUBFLOOR, from the top & below in your moist but never wet basement... although if you're having issues with the upstairs, the problem is not originating in the basement.

Back to my original comment... your location may be a clue.
Hard to tell by your pics... but it doesn't seem as if the floor is "shrinking" which could cause the faces to cup, but only slightly... and there would be visible "cracks" between the boards.
More likely that the floor is expanding (across the bottoms) as suggested.

The dehumidifier MAY show results, but I wouldn't count on seeing them all of a sudden.
As far as sanding the floor (again), that wouldn't be a prudent approach until the PROBLEM is ascertained & solved... and even after that, you'd want to allow the floor as much time as necessary to dry out before sanding... because as it dries, it will undergo "changes" again.