I have an Emperor 1 Japanese maple (in hardiness zone 4) which I planted 18 years ago and it has done well, fast growth, no issues till recently.

I noticed last fall that some of the leaves didn't fall off, unusual. Then this spring about 25% of the tree did not leaf out, so I cut out those branches. Then I noticed the dead bark in the crotches near the tree base, pulled off pealing bark and revealed these dead spots on the trunk that appear to be dry and healing over. This situation is in 3-4 crotch locations.

Any ideas what caused this, and will things improve on their own?

Where are you located? More and more JMs in the west are being infected with Verticillium wilt.

I'm in Minnesota.

Reading up on verticillium wilt, I am almost certain that that is what is striking a Korean maple in the backyard. It had large sections of dieback and half the tree rapidly dying. It would appear to recover then suddenly have large die back. A few years ago I cut it down to the ground, it resprouted to the side of the old trunk and is now about 6 feet tall and not yet showing disease.

Anyway, the Japanese maple is showing different symptoms, no yellow leaves noted in the growing season last year and this bark damage must have been going on for many years under peeling bar unnoticed by me.

I will look for the stained pith typical of verticillium wilt since I just cut down branches.

Reading more, I think it must be a canker of some sort. Every source is recommending to remove the tree.

I did notice something that looked gummy in one of your photos.