It's become clear now as to what has happened to Lewandowski.
Lewandowski developed a health issue during / following the world cup.
He has recently stated that in 2023 "there was a moment when I felt not only mentally, but also physically weaker" and that he went on to do "physical tests".
This is a clear admission that his health took a 180, and the reasons seem to be unknown.
“In 2023, there was a moment when I felt not only mentally, but also physically weaker. That year, all the bad and negative things seemed to come together in one moment. Everything accumulated,”
“Of course, I also made mistakes. I fell into a kind of current and it took me too long to get out of it, to return to normality, and in football it is not so easy to do it in a week or two weeks.
“During that period I did physical tests and individual training in my free time and it turned out that it all looked good and it also gave me a lot of answers. I could see that I am moving forward physically.
“I can look back and this is an indicator for me that shows me that there are still good times ahead, but in 2023 there was a moment when my spark went out.”