  The spin-orbit interaction is the key element for electrically tunable spin
qubits. Here we probe the effect of cubic Rashba spin-orbit interaction on
mixing of the spin states by investigating singlet-triplet oscillations in a
planar Ge hole double quantum dot. By varying the magnetic field direction we
find an intriguing transformation of the funnel into a butterfly-shaped
pattern. Landau-Zener sweeps disentangle the Zeeman mixing effect from the
spin-orbit induced coupling and show that large singlet-triplet avoided
crossings do not imply a strong spin-orbit interaction. Our work emphasizes the
need for a complete knowledge of the energy landscape when working with hole
spin qubits.
