Commercial fruit production outside of wine grapes in this country spends little time on the question of brix because the market is mostly about yield per acre and appearance and storability of fruit.
I can’t verify it with research, but it seems irrefutably logical that in humid regions, years of mulching trees, especially with wood, gradually increases the amount of available water in the soil significantly. The result is a gradual reduction of brix during years of ample rain during the ripening process. Big fruit with big watery cells. Also trees with excessive vegetative vigor.
It’s something that anecdote enforces in my own orchard, so now I let sod grow to the trunks of trees once they are adequately bearing trees.