Defrost Mode

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Why would the 2nd stage heat not activate when in the defrost mode? I know that 2nd stage comes on if the thermostat set point is raised +3 degrees. In fact, I do not recall seeing 2nd stage heat activate when heat pump is running in 30 degrees or below temps. In addition, heat strips do work in emergency mode.

Robert Shaw 9420 Thermostat
1st Stage is set at 1 degree differential.
2nd Stage is set at 2 degree differential.