Clues seem to be VST 2 (not 3) and Reaper.
Tested that it works in Studio One with Pianoteq VST 3.
I'd, if you can, figure out why you're using VST2 and not VST3 Pianoteq plugin... but after than, it may come down to something to do with the way Reaper works.. not sure of a workaround - but at least confirm it's not a prob in VST3 in a different DAW..
Maybe other Reaper users might be able to help more. Check settings in Reaper (I remember noting that it has a couple of 'veiws', like listing VST controls in plain scollable form - so IDK if something to do with that might be interrupting normal view, at least for VST2 plugins??)
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors