Do I understand correctly. Do you want different continous/discrete behaviour between your plugin GUI and plugin automation? I don’t understand your example.
I just tested in Reaper. For me the discrete functionality is working fine in VST3, but not with VST and AU. I implemented the AudioProcessorValueTreeState and added a Parameter with NormalisableRange with an interval value. That probably needs some bugfixing…
top AU, middle VST, bottom VST3