Slot-frame switches from fill to fixed when I add a new child

I've created a nice container-component that has a slot. It consists of a parent (height fixed) and a child (height fill). Whenever I add an extra component to the slot, the slot jumps to height fixed, but I can't think of any reason why that's necessary or desired. I now have to manually reset it to fill, which is not helping with productivity. What I want is that the slot just keeps the set height property as is defined in the container-component, because that was my intent in the first place. I could only find an old closed post about it that seems related, but closed: