This is a map (revealMap 128 was used) of 1523 seed. Now can you see that something is wrong happening? Like, there are two black areas where revealMap gives up, and this:

This area of world is located here:

I can definetely see that something is going wrong.

Then, I created a world with the exact same seed. This world map greeted me:

Now, the spawn region is completely different, and this time it really belongs to 1523 seed.

This means that the first world I ran was with the spawn region that isn't 1523.

Now, no matter how I tried to reproduce the bug, it was impossible for me.

Though this is still pretty weird.