Did you collect the Chinese Chestnuts? The longer the chestnuts lays on the ground the probability of mold increases. If it is damp and the limbs of the chestnut tree hang down to the ground or close to the ground - this increase the chance of mold. High grass increases the chance for mold.
If your tree allows the burs to open up on the tree first and then drop chestnuts, I have a suggestion for you. Put a tarp on the ground one evening and use rocks to hold it firmly in place. Next day come collect the burs and nuts that landed on the tarp. You are reducing ground contact. An alternate plan is to put the tarp down and immediately take a rake or piece of conduit and work on the limbs to get burs and nuts to fall right then and there. This reduces ground contact.
I only had mold issues one year and I learned to collect nuts off the ground more often. Some people rake up all burs so they know what has dropped recently.
I soak my chestnuts over night to hydrate them after collection. I use two fans to air dry them on a tile floor with two towels to hold the nuts. I like to put my chestnuts in the fridge dry without anything for the first month. Peat moss holds moisture and I have found it promotes mold for me. I use the long fibered sphagnum moss to reduce mold.
Some people use Clorox diluted in water - not me. Some have stated they use fungicide - but I have not followed up on that.
I like hydrated dry nuts in the zip lock bag to reduce mold. I like chestnuts collects ASAP once they hit the ground.
I hope something I have shared helps you see a way you can reduce this problem. Hang in there - you will get it going your way.
The shade is a contributing factor no doubt. The tall grass on the bank is not to your advantage. If you can weed eat it, that will help.
Sounds like you are working at collecting under that tree fairly often. How many nuts do you hope to collect? If you wait until prime time and use a couple of tarps, you can rake up a high number.
I am glad to hear what you are doing. We need more people collecting chestnuts from their area. When I first started collecting, then I began to locate more trees because my awareness and knowledge was better.
I have stopped collecting as of 2-3 weeks ago. I didn't want more than I wanted to tend to. The tree didnt do that well this year, got mostly single nut in the burr.
Last year had several with double and a few triple nuts.
Always factor in for some losses. Nature will claim some of our weak seedlings too.
It has been my experience that the burs that are shaded or on the north side of tree can be the last ones to drop. You can find some great chestnuts in those situations. My advice is next year - factor in for some losses. It happens to all of us.