Can you please have a look at https:/perfecthealthdiet.de?
I have two types of posts: “normal” posts like https://perfecthealthdiet.de/kalzium/, and recipes (e.g. https://perfecthealthdiet.de/rezepte/knochenbruehe-bone-broth/). Both have featured images. For both, the featured image shows up on the blog page. For recipes the featured image also used to show up in the single post, but it does not appear any more. For “normal” posts everything is still fine.
I’m using a recipe plugin (WordPress Ultimate Recipe) that creates the cpt for recipes. I’m not posting very often, but I’m sure that the featured image was displayed when I checked last time (about 4 weeks ago). There was no WPURP update during that time, but I seem to remember a GP or GPP update.
Interestingly, when I set the location of the featured image to “Above Content Area” (Customizer – Layout – Blog – Featured Images/ Posts) they do show again.