The thumbnail is based upon the first chosen for uploading image. I have performed some tests on our local environment using 4.2 1650:
1. Created 3 galleries.
2. In two of them I have uploaded the same 4 images in the same order. Both of them were using the first uploaded picture as a thumbnail.
3. I have uploaded the images in different order and again the same scenario applied - the first image was set as thumbnail.
If you experience somehow different behaviour please record a video and provide further details about your project and setup.
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