The 'Sync Product Variant Images' plugin appears to be hiding them. I can't be sure as I don't have access to your site, but it's definitely some JS in the code injection area, not CSS.
You can temporarily remove the following line of code from Settings > Advanced > Code Injection to test this:
<script src="/s/plugin-sync-product-variant-images.js"></script>
If the thumbnail images re-appear then I recommend you contact the plugin supplier to understand why.
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