First clicked image smaller

[au lait]

Affected version

2.3.9

As you can see in this test thread, the image that is clicked on first is reduced in size in the display.
Although it has the same dimensions as the others...
If you use the <> keys to scroll through everything, everything is fine!

https://xenforo.com/community/threads/post-with-lots-of-big-images.234325/

video evidence