My ginkgo is 21 years old. For nineteen years the tree was a glorious gold in the fall. The last two falls the leaves have not t
urned gold byt dropped within a few days when they were all green. Two years ago the landscaper mounded mulch around the base of the tree for a heighth of 8 - 10 inches.
Mulching should not affect leaf color but make sure the mulch does not tough the trunk. When mulch is in contact with bark, it can lead to bark rot and mice tunneling through the mulch to chew on bark. Where I live, we have lately been having short autumns; winter temps came the second week of Nov. including overnight temps in the teens. This did not allow some trees and shrubs to fully go through the winterization process. Thus, the ginkgo down the street dropped green leaves.
There are several factors that are important to consider for growing conditions and management. Primarily soil and water management, nutrient deficiencies can develop over time, drought and other physiological stress factors, soil grade changes, compaction, trenching, fuel spills and other impacts can interfere with normal seasonal processes.
Since the mulch is the only aspect of the growing conditions that you mention, and the only factor that you indicate has changed, I can only comment on that issue. And your hunch is legitimate about that much mulch against the tree trunk being suspect as a factor in the changes. Mulch is a good thing, but too much and especially too much against the tree trunk can be detrimental.
If mine, I would pull away the mulch from the base of the tree, to achieve a 1 foot radius of mulch free zone around the base. I would also take off a layer of that mulch, take it down to 4 inches thick instead of 8 to 10. You can redistribute the excess mulch in other areas of your garden, or start a compost pile.