Troubleshooting help: Issues with the Post Element

I’m having some weird issues with the Post Element on one specific development site, which are not happening on my other sites. I thought it must be a plugin conflict but deactivating all plugins didn’t resolve the issue, and anyway the plugin list is almost exactly the same. In fact, I cloned the site where the post element is not working from the one that it is working on. Both hosted by the same host.

Summary of the issues:
-All featured images are displaying as a small square, no matter how I try to change the grid image ratio or image height and width settings
-The gap between images can’t be adjusted
-Adding filters makes the gaps collapse and also the image heights collapse, although they reappear if you select the filters, still with the small squares and big gaps

I’m going crazy and this is the one thing holding up the completion of this build. I do have some custom css on the site, but I can’t see what would cause this issue.

The site can be viewed at:

Screenshot showing my portfolio (that’s 8 images squashed into little rectangles):