This is happening because items in HTML are set to display: static by default. This means that when their dimensions change, they will push all other elements around them to create their own space, sort of how position: relative works.
Your fix;
Set the Collection Item with the class Collection Item 6 to a max height of 315px only on desktop. Set to none on mobile
Use this piece of code to make sure that the content shows up on top of the other cards when they expand. Put this in the footer code section in page settings;