Variant images (covers) missing / not showing i

I have a Shopware 6 system running with multiple products. Some of them has variants. These variants are shown as I would expect.

Now I switched the displayType to image for this variant group. And now some of my variants do not show the image in the variant picker on the right (in frontend) while the cover images to this variant are existing and gets shown when selecting this variant.

Debugging the media variable in configurator.html.twig where the variant image gets rendered show on the variants with missing images false – on other variants the object of the media and the images gets shown. I cannot find any difference between variants with and without image – both have images applied and set the correct one as cover image. Also when I try to change the image on a variant which shows an image right now the image in variant picker do not change.

I also tried re-generating all thumbnails in the whole project via CLI. Also no effect.

Clearing caches did not do anything.

I also saved one of the missing images in variants and reexported from Photoshop for web (JPG) und replaced it. Also no effect.

How can I update the variant images when changing the cover-images in variants do not take any effect?