I was told by Uhe that Diva supports true Poly AT from any Poly AT equipped keyboard. However it does NOT work in GP as far as I can tell. I tested this extensively. I confirmed that Poly AT works using the free version of another popular VST host so it appears to be a GP issue.
Can you please check if the issue is related to all VST3 plugins in GP or is it only related to Diva/U-he?
To do this you can check with a different VST3 plugin (and also compare it when loaded under another host).