Distributed VST3 plugins don't receive parameter updates in the processor

[olilarkin]

Issue type

VST bug

Description with steps to reproduce

• Load a VST3 plugin that uses the recommended "distributed" approach where Controller and Processor are not directly connected, e.g. VirtualCZ...
• Change parameters in the plug-in UI/Controller.
• The processor/DSP is never updated, since parameter changes are not received from the controller.

Note: plugins that use the SingleComponentEffect base class don't have this problem in MuseScore
Note: the bug can be reproduced using the iPlug2/IPlugInstrument example here. In order to compile the "VST3 Distrubuted" target, you can apply the attached patch.
iPlug2-22-34-38.patch

Note: I believe the problem will also be present in the VST3 SDK examples

Supporting files, videos and screenshots

https://drive.google.com/file/d/1JVsk-WDEkbFBPBZk7kyIk2KFuo83s11R/view

What is the latest version of MuseScore Studio where this issue is present?

4.4.2

Regression

I was unable to check

Operating system

macOS 14.6.1

Additional context

No response

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

[olilarkin]

VST3 docs... https://steinbergmedia.github.io/vst3_dev_portal/pages/Technical+Documentation/API+Documentation/Index.html?highlight=messag#communication-between-the-components