The problem: Off by one error for negative coordinates. Source: Behaviour of rounding coordinates (doubles) after deform.
The off by error came down to rounding using casts (int) and rounding using Math.floor()
(int)( 1.8) = 1
(int)(-1.8) = -1
(int)Math.floor( 1.8) = 1
(int)Math.floor(-1.8) = -2
Looking at the original WorldEdit implementation a Math.floor call is present too. It was missing FAWE which resulted in the bug.
Co-authored-by: Alexander Brandes <mc.cache@web.de>
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