margin-top is deforming on onderhoud page between 541 to 640px?

I have removed max-height from the class:

.flex-container-onderhoud-page-section1 {
	
	  width:100%;
	/*max-height:500px;*/
	display:flex;
	flex-direction:column-reverse;	
  margin-top:1rem;
	
}

and the problem is solved .

I don’t understand why max-height is causing problem with margin-top.

there must be some answer for this issue.

thanks