Hi,
I was not aware of the fact that for perfect compatibility of a VST3 with its VST2 version, it is necessary to use JUCE_FORCE_USE_LEGACY_PARAM_IDS=1 when building the VST3 , otherwise all automation data is lost when switching between them.
[ Guideline for replacing a VST 2 plug-in by a VST 3 plug-in - VST 3 Developer Portal ]
Maybe it could be added in the documentation for JUCE_VST3_CAN_REPLACE_VST2 ? Because once the plugin is released, it is too late to fix that…