Yes @mikey, for the studio I will discuss this & the other topic ([studio]matrix cell rendering issue) with the team to see if it is a bug to declare or not.
But as a workaround, you can give a fixed height value (for example 400px in your case).
The same for the width you can assign it a fixed value.
Each time you can increment/decrement the values and automatically the changes are seen in the studio, just to check if there is a gap between each items (vertically & horizontally).