You are right!! Disabling the Woocommerce Blocks plugin and adding a short code solves the problem. See the next example-page (made with a deactivated Woocommerce Blocks plugin): https://vruchtbareaarde.nl/webwinkel/proefpagina-zonder-wc-block/ (the two right products have a secondary image)
But then … I activated the Woocommerce Blocks plugin again and started with a fresh “category” with some products. In this case (with an activated Woocommerce Blocks plugin) the secondary hover effect was working as well. See: https://vruchtbareaarde.nl/webwinkel/product-categorie/proef-wc-blocks/
DISadvantage of the second option (with activated Woocommerce Blocks plugin) is that the URL is less SEO-friendly. I would rather have an URL without “product-categorie” in it.
But then I lose the easy styling options of the Woocommerce Blocks plugin.
Any advise from your side?