Hi Jornes,
that’s because we still have a container on the canvas around the actual element (.bricks-container-inner) and that’s the reason why this doesn’t work especially with CSS grid, because CSS grid is not implemented at the moment.
For simple things like colors, sizes etc. it already works directly and without problems.
Until we have implemented grid you have to take care of it yourself. How to do that, I have explained in this thread: CSS Grid does not display correctly in the editor, but works in the frontend
A bit hard to follow… but i think you´ll get it 
Best regards,
timmse