Japanese Maples can often suffer from root rot. You may be over watering the plants, or they have no ability to drain freely out the bottom. You may need to add some vermiculite or a soil additive to add air to the soil.

Also, the size of the tree is quite large for the size of the pot. Perhaps the tree is becoming rootbound in this pot and is starting to strangle itself. I would suggest either putting it in a larger pot, or planting it in the ground. If you do so, you may want to have a look at the roots and cut any that may be girdling the tree before planting.

Japanese maples are very susceptible to damage from late spring freezes. I lost one for sure, possibly three more after they broke dormancy in a warm February followed by a month of real winter in March. The cambium layer of the main trunks were completely to partially frozen and killed. As the bark has begun to flake off, it reveals whether the tree was completely girdled with loss of the cambium or if there is a remnant connection to the roots that might allow them to recover.