Custom expression used multiple times as input to another expression does not work correctly.

Given a custom expression that takes a vector and returns a new vector swapping the Y and Z dimensions, if I use that custom expressions on two distinct input vectors (in this case position and velocity) and pass the results to the cross product function I always get (0, 0, 0) which is crazy. If I replace one of instances of the custom expression with an identical copy with a different name everything works fine.