Grid system showing gaps

I've been trying to figure out how to make a responsive grid, with some elements spanning 2 tiles,
but I've been running into the issue that there are gaps showing up next to the spanning elements.
Is there a good way to fix this? Any tips would be greatly appreciated!

Here's the Codepen:
https://codepen.io/aldouss/pen/yLZZZPa
image.png
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