here are some related internet posts about VST3 and program change.
shorturl.at/GLV16
etc…
you can find many more if you google.
VST3 uses a parameter based approach in the VST3 api, totally different then the VST2 api. They are called “program lists” I believe. This has been the case for a long time, and up until recently, VST3 plugins such as VePro and Patchworks were able to translate the program lists into actual PC messages to be passed on to sub hosted plugins. Apparently, with Dorico 3.5, they are not able to do it anymore. So something has change in Dorico 3.5 that is just enough different that this necessary hack has become broken.
I don’t own Halion and really don’t understand what you’re going on about with Lua, etc… If that helps you figure out what exactly changed in Dorico 3.5, then great!