All of our stores are missing since updating our website to PHP 7.2

The CSS file loaded in HTML of your site is the netdna one, not the local one.

The code simply isn’t there.

If you disable the CDN, then you can verify if the code is correct. Or if possible, move the CSS file to the CDN yourself?

I got the caching of the style.css file from the CDN corrected and the changes from the code you provided look to be reflecting on the website. However, the thumbnails in the results below the map, while no longer distorted, are now really small in size. Can anything be done about this?

Thanks!

You can simply define a format in the CSS by replacing auto with the pixel format you want to use.

For example.

#wpsl-wrap #wpsl-stores .wpsl-store-thumb {
height: 150px !important;
width: 52px !important;
}