In the builder structure, a component slot is an additional div through which the elements placed in it do not behave as they should as child elements of the actual parent.
For example:
Flex Parent with gap > (SLOT) > Child1, Child2
In the editor, the gap will not separate Child1 and Child2 because they are not direct children of Parent because of the Slot div in between. Fortunately, on the live page, where the Slot div is no longer rendered, everything works fine.
It would be great if the builder reflected this too, but I can imagine it may be tricky to fix.
Likely solution #1: Maybe making the Slot elements inherit some parent properties would sufficiently solve this problem, it worked in my simple test.
Likely solution #2: Making the slot element like code block so it is element rendered on the page and can have styling, with option to render without wrapper, and option to not render at all if empty.