I build this kind of grid for customers and it’s almost always the same problem.
Users will upload images with white borders, some more, others less.
Without inspecting the code i can only assume, this could also be, like you said, conflicting frameworks, where their CSS keeps fighting each other for display supremacy.
Edit: i think you’re right, this looks like the failed attempt to add 40px margin every x amount of cards, while trying to match the needed margin for a new row.
A forgotten media query, it still has the margins from the 5 col grid for the bigger screen