Here is an unexpected behavior with fma contractions when done in a wild manner:
x1y2 - x2y1.
We found that this is evaluated as fma(x1, y2, -x2*y1) in precise model on apple M1, that is one multiplication and subtraction is done using high precision the second multiplication is rounded. As a result this expression is not zero when x1 == x2 and y1 == y2.
It seems the contraction should not be applied to anything more complicated than a * b + c.