Not a physicist, but here are a couple Wikipedia excerpts I found:
From page “Conservation of mass”:
“In general, mass is not conserved. The conservation of mass is a law that holds only in the classical limit.”
From page “Neutrino oscillation”:
“Neutrino oscillation arises from mixing between the flavor and mass eigenstates of neutrinos. That is, the three neutrino states that interact with the charged leptons in weak interactions are each a different superposition of the three (propagating) neutrino states of definite mass. Neutrinos are produced and detected in weak interactions as flavour eigenstates[a] but propagate as coherent superpositions of mass eigenstates.[23]”