Hi @friese,
thank you for your question. I would strongly recommend that you try to avoid using the TVector3 legacy class and instead try with ROOT::Math::XYZVector.
While I tag @moneta for some further insights, I invite you to have a look at this thread from a few years ago, where a similar issue was discussed: Rotating vectors with ROOT::Math::XYZVector.
Cheers,
Marta