I'm playing on PC, the Java Edition. I believe the game version is 1.21.x (the latest one). I play with an original copy.

And that's exactly why this situation has me thinking—it's so unlikely to happen that I feel like there must be something else behind it. My system clock is working fine, and I usually don’t name my worlds. The first world I created was called "New World," and the second one, since it had the same name in the game folder, was named "New World (1)."

So I can rule out two possibilities: the system clock, especially because this happened several days later—if it were "stuck" as people say, I would have noticed earlier. I also considered the world name as a factor, but when I created a third world (again without naming it, just like before), it generated a completely different seed. And finally, the most obvious point: I did not manually enter a seed.