When spawning an actor and setting the relative location, the sign of the Y value of the FVector is flipping from negative to positive. Below is a screenshot of the code and the output log.
Is it possible that the Parent actor that you are attaching this to is simply rotated?
I’m not sure if SetActorRelativeLocation takes the transform of the parent as its identity, or if it continues to use world space.
However my assumption would be that it takes the parent as identity, as if you want to set something 100Units “behind” the attach actor then you would want that relative “behind” to remain consistent regardless of orientation.