Some of the branches may have died. To test this, lightly scratch a small area of the bark with your fingernail to see what color is beneath. If it is white or green, and moist, the branch is still alive. If it is dry, brown or tan it is dead. We are having a cold spring and your tree may be slow at completely leafing out. However, a tree that is producing more seed than it has in the past is sometimes a sign of stress.
I can see from your first photo the tree is mulched too deeply. The trunk should be flaring out at the base( like a persons neck flares out to their shoulders). When a tree goes straight into the mulch or soil with no flare it means it is too deep in the ground and the root system doesn't get enough oxygen exchange. This causes gradual tree decline.
I think this may be the cause of your tree loosing some branches. Others causes are disease, or lack of water, or too much water. There are other root issues, like girdling root, that can cause branch die back. Voles or mice chewing the bark below the mulch, especially in winter, can cause dieback.
Correct the mulch by pulling it back at least 4-6 inches from the trunk- if there is a 'flare' at the soil level that is good. Check the mulch to see that it is no deeper than 3-4 inches all the way around the tree. Prune back any dead branches. Give a slow watering to the tree when soil is dry and we have droughts in summer and fall. Soil should be damp, not soggy, down to about 6 inches before you stop applying the water.
This tree still has live branches and with good care may sprout more over the next several seasons. If that is too long to wait, you could replace the tree.
I am including the Tree Owners Manual which explains how to water, prune and mulch trees, and also explains how to correctly plant trees should you decide to replace yours. I hope this helps with your Japanese Maple.