The trigger input is based on the wether or not a cable is plugged in and currently has config associated with it, and in Dual Mode it is shared and applied to both Quantizers.
While it’s probably technically possible, the config UI would become more complex and would need to rely on some hidden options.
You could achieve this by using a separate S&H module (Like Noise Tools 1U) connected to OUT A then leaving the TRIG input unconnected.
You are correct in how it should behave. If you have nothing attached to the trigger input, sweeping the voltage on the PITCH input should change OUT A to a new quantized value as they are reached (try using a manual offset or LFO to see this work). If you have a cable attached to TRIG IN then OUT A will only update when a Trigger is received AND the new value is different than the current value.