It's definitely hard to spot sometimes.
Spritesheet is over the 3MB limit unfortunately. I can see it with any initial planet seed (from when the page first loads) - set pixels to 400, and export the spritesheet as 20x21 tiles.
asdfI tried to record the in-browser version next to it, not necessarily as a direct comparison, but for the sake of having a smooth animation as a visual benchmark. If you stare at the line between the two, it's more noticeable after a few loops.
I think I got a good part slowed down that shows it pretty well.
asdf2
If you look to the bottom left cloud cover, where it starts opening up to a circle above an ellipse, and watch as the end of the ellipse opens up, that's where it jumps - that moment where it opens up. I admit it's a very slight jump, as mentioned in the first comment probably only 1 missing frame, but after staring at the planets for a while and watching other 60fps animations moving around it, it starts to become a lot more evident.
I also tried generating a double-wide 40x11 sheet and that seems to at least reduce the frequency of the jumping, so that may be a workaround in the meantime if I regenerate all the planets at a smaller size with fewer rows.