This does seem to fix it!
A couple notes to future visitors:
If you are attempting to make a “mono only” plugin, I believe the following options are necessary in AppConfig.h:
#define JucePlugin_PreferredChannelConfigurations {1, 1}, {0, 0}, {0, 1}, {1, 0}
The 0’s are necessary because the VST3 validator (if you’re running it) disables the master bus. See related topic here:
That’s all. Hopefully this helps someone in the future!