Thanks for reporting this. I’ve had a look at it and this is caused by questionable behaviour on the plugins part - it’s writing its output audio to both the input and output buffers that Cantabile passes to it which is trashing Cantabile’s copy of the input signal causing it to leak back into the audio stream even though it’s bypassed.
I don’t think there’s anything in the VST spec about whether a plugin is allowed to do this, but it seems very questionable to me.
Only way to fix this (besides getting the plugin fixed) would be to make a copy of the input signal before passing it to the plugin. Shouldn’t be hard and I’ll special case it for this plugin. I’ll try to fix it for the next build.
FYI: If you look in Compatibility.ini in the same folder as Cantabile.exe you’ll see a new compatibility flag you can set for any other plugins you find with this issue.