Found the answer to this problem in another post here:
https://forum.cockos.com/showpost.ph...3&postcount=11
For some reason the required code was commented out in iPlugVST3.h
Wavelab requires setState and getState to copy the plugin settings from the active plugin to the rendering plugin copy. Without this Wavelab renders using the plugin's default settings - i.e., it constructs the plugin but doesn't copy the settings.
It was hard to catch this error on some plugins because they APPEARED to be working when in fact they were rendering with their default settings.