Stain/damage from gym flooring on hardwoods?
Hello there. For the past two years, I have had 3/8" thick gym flooring on my hardwoods in one of my bedrooms. When preparing for an upcoming move, I lifted up the flooring and layers of "protection" that I had underneath the gym flooring, and I found these stained areas of my floors. Underneath the gym flooring, I had a layer of plain brown "butcher"-type paper as well as a layer of cork underlayment. I did these two layers based on Reddit research at the time. The staining (?) seems worse underneath where I had a heavy stationary bike (in the far back corner in the 1st photo), but there aren't really any explanations for the other heavily stained areas. The entire room had the paper/underlayment/gym flooring covering the hardwoods, but these are the only areas that were affected.
Can anyone lend some insight into what might have caused this? I find it hard to believe that any of the black coloring from the gym flooring could have gotten through the cork underlayment to stain the hardwoods. Could it be from moisture under the floors?? And are there any products that could clean these stains, or do I just need to replace the flooring?
I appreciate any knowledge or advice you can share!!
Here is the gym flooring that I had:
https://www.sparksfitness.net/products/8mm-pre-cut-rubber-flooring-rolls
And the cork underlayment: