Merge mode with a pattern of a carpet succeeds, even in survival mode with inventory containing nothing but the pattern:

This is caused by WorldWrappingMutator#getInAreaTarget calling WorldWrappingMutator#isInside, which checks if the passed vector is inside the word bounding box, rather than the in-area one. Unlike WorldWrappingMutator#setInAreaTarget, which does it properly by using final Vec3 actualTarget = getInWorldStartPoint().add(inAreaTarget);

I don't know if other IStateAccessor implementors have the same problem or not. I'm also not sure how correcting WorldWrappingMutator#isInside will effect other calls to it and WorldWrappingMutator#getInAreaTarget or WorldWrappingMutator#getInBlockTarget outside of pattern mode classes.

C&B 1.18.2-1.2.90; Forge 40.0.46; No Optifine