I have an organic chemical (can't say due to an NDA) that has a nominal melting point of 28 °C. When I raise it above this temperature, it melts, but then it will stay a liquid indefinitely at lab temperature (≈22°C), and in the fridge (4 °C) for at least a week. When put in the freezer (-20 °C), it will freeze, and then stay a solid at lab temperature indefinitely.
Is there an explanation for this phenomenon? I'm assuming this may have something to do with the kinetics of the crystallization, but I've never seen something with this level of hysteresis.