When a spin-1/2 particle is placed in a magnetic field that is strong enough and varies slowly enough in space and time, it will become polarized and its spin will either align or anti-align with the magnetic field direction. Let the magnetic field be denoted by $\mathbf{B}$ and the spin by $\mathbf{s}$ . Then, under this condition, the potential energy becomes
$$ U = \pm\gamma|\mathbf{s}||\mathbf{B}| $$
where $\gamma$ is the gyromagnetic ratio (which can be negative) and the upper and lower signs correspond to the spin-up and spin-down states, respectively.
I believe all of this is standard quantum mechanics. When the magnetic field exhibits a spatial gradient (which isn't too large), then the field will induce a force on the spin-1/2 particle. This is the basis of the Stern-Gerlach experiment. And, in particular, the SG experiment indicates that the spin-up and spin-down states are (at least roughly) equally likely when the particle source is initially unpolarized, or polarized in a direction perpendicular to $\mathbf{B}$ as observed in sequential SG experiments.
However, based on the potential energy function given above, it is clear that the anti-aligned state has a lower energy for a given value of $|\mathbf{B}|$ (assuming $\gamma > 0$). So should I be surprised that two states with different energy levels should appear to be equally likely? Is there any preference for the aligned state to eventually transition to the lower energy anti-aligned state within a constant magnetic field? I've never seen this issue addressed in QM textbooks or elsewhere.