I added a vendor prefix and fixed some images, and then grouped the shorthand rule background: for color and without repeating. This captured some of the images, but not all, so I noticed that this was due to the size of the image you have, I used the same image (which worked) for all the images, and it worked.
So the aspect ratio of your images has something here, and I think because background-size: cover; trying not to distort the image, ruining the aspect ratio.
This is where my violin works ... just rework your images so that they have an aspect ratio that would fit its container correctly.
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