Any chance you updated macOS itself?
What I don’t understand is why it kept working but just worked differently unless it was pulling in some bogus settings from somewhere
As mentioned earlier, I really have no idea where the problem came from.
From today’s perspective, I can rule out GigPerformer.
It is now also known that the VSX release v6.0.5 contains minor bugs - nevertheless, it should produce identical results across all plugin formats.
I suspect it actually had something to do with the previously installed beta.
However, Steven Slate is very committed to ensuring that VSX and GigPerformer work perfectly together.
Here is a step-by-step tutorial how to do a null-test: