The short answer is YES this is a normal situation for a depowered B737. When there is no power on the A/C there is no hydraulic pressure to the rudder to prevent movement. There is a damping effect of the fluid left on the lines but there is no "fluid lock" of control lock in the system.
The procedure at my company is to leave at least 1 electric "B" pump on in windy conditions to prevent such flapping. Unfortunately this is to no avail when there is no power on the A/C.
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