FWIW the VST3 version of my plugin (also made with iPlug) behaves the same in REAPER, always snaps back to first (default) preset entry though the parameters do change. (the VST2 version behaves correctly in this regard)
I always thought it's a bug in iPlug so I stopped bothering at some point, also I don't have another VST3 host currently to test / compare with.
It's here:
http://www.vst4free.com/free_vst.php?id=2483