Not necessarily. A point that may be overlooked is that ice doesn't absorb the 'microwave' radiation as effectively as water. So more power may be reflected until such time as enough ice melts to increase the rate of heat input accepted by the peas.
And I'm far from sure the absorbtion is due to a resonant line of the water.
So experiment may be a better guide than some of the theories I've seen here.
Slainte,
Jim