  Spin qubits in gate-defined silicon quantum dots are receiving increased
attention thanks to their potential for large-scale quantum computing. Readout
of such spin qubits is done most accurately and scalably via Pauli spin
blockade (PSB), however various mechanisms may lift PSB and complicate readout.
In this work, we present an experimental observation of a new, highly prevalent
PSB-lifting mechanism in a silicon double quantum dot due to incoherent
tunneling between different spin manifolds. Through dispersively-detected
magnetospectroscopy of the double quantum dot in 16 charge configurations, we
find the mechanism to be energy-level selective and non-reciprocal for
neighbouring charge configurations. Additionally, using input-output theory we
report a large coupling of different electron spin manifolds of 7.90 $\mu$eV,
the largest reported to date, indicating an enhanced spin-orbit coupling which
may enable all-electrical qubit control.
