it appears, that RotationSuperoperator with sympbolic phases "-x" & "-y" returns a non-numerical matrix, which cannnot be used for Amplitude calculations:
TransformationAmplitude[SpinDynamica returns the erromessage:
opI["z"] -> opI["y"],
NPropagate::nonnumeric: A non-numeric matrix was detected. The requested context does not permit Operators or Superoperators with symbolic coefficients.
Setting the phase numerically (pi) circumvents the problem. I attached a notebook demonstrating the issue.