OK, this is a weird one that has been bugging me for a while. I just recently discovered how to reproduce.
With just a Waves stereo plugin (vst3) on a mono channel, every thing works fine. But when I insert a Steinberg stock plugin that has side-chain capacity (compressor, gate, expander etc) anywhere in the project, the right (unused) channel of all stereo waves plugins (that sit on mono channels) start to peak with levels of 380dB.
This seem to only affect waves plugins in conjunction with Steinberg side-chain enabled plugins. It might be a Waves issue but it only happens when I also use Steinberg stock plugins.
This doesn’t matter much for linear type plugins (eq) but seriously destroys everything dynamic or pitch etc. Using a mono waves plugin solves the issue. But it would be easier to work with just one plugin instead of mono and stereo versions, with vst3 it should just work, right?
Create empty project.
Create 2 mono channels.
On channel one insert a stereo waves plugin, like REQ4 stereo or Stereo Deesser.
On channel 2 insert Steinberg compressor.
Watch the meters on the waves plugin.