Taco zone valve power head issue

Got up this morning to find an over temperature condition (temperature way above thermostat setting) on the first floor at home. At first I thought it was a thermostat or a flow control valve (FCV) issue but it turns out that the heat motor piston in the power head (PH) for that zone is stuck in the "expanded wax" position. Due to the FCV spring pressure, it was one hell to remove the PH from the FCV housing even after disconnecting control voltage and waiting five minutes.
It's a Taco 555-102 PH. With it removed from the system, the piston remains in the expanded position. Am I missing something or can the wax in the heat motor change properties like this?

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