This is an extremely common bug that is caused by the game not deleting old files properly when you delete a world (due to improper client-server synchronization, a bug that can be fixed in a few lines of code):
(I've seen several other threads about the same thing within the last few days)
Also, this has nothing to do with the seed and cannot be reproduced (even if you reproduce the bug it won't look the same since what you see is actually the result of two different seeds, with the buggy terrain being generated with a different seed, and this also depends on what you explored in the old world).
1. If you're not, I suggest that you use the random feature of Amidst (see my sig.) to search for an island spawn world.
2. Use the seed displayed by Amidst (hit Ctrl-C) to name the world when you create it.
(this prevents using the same name twice)(If I create it again I add a letter to the name a,b,c, etc.)
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