I guess I am a bit confused by the "confusion" around this topic. There is definitely believed to be another "developmental stage" psychologically for most people that occurs in the time frame we call middle age. This is parallel to various stages (teen, 20's, etc) that we go through. It is a general guideline and idea, not an absolute. And this is the time most of us go from viewing ourselves as eternal and young to more accepting that we are at a certain point and that we are not "young," whatever that means to most, anymore.
But what happens in this stage is not a singularity. Some people mature even more or decide to give back and focus on others. Some go through depression. And I believe this is where deep issues and fractures tend to stop being able to be papered over. It makes perfect sense, if I am always thinking that in the future I will do xyz, then suddenly I start to see future is not forever the issues underlying suddenly become acute.
What we tend to call MLC is a mixture of multiple psychological issues and disorders, and although the patterns are the same the exact details are as varied as the individuals. Which is what is said here in many different forms.
I don't think we need to "study" MLC, rather we need to create societies that value self awareness, empathy and growth over "accomplishment," rigid role models, and material wealth. Because if we raise children who are self aware, caring, flexible, inclusive and kind they can handle what life brings and continue to cope and thrive. I know my wife for one definitely did not develop the skills in her childhood to do that.
MLC is sadly predictable.