N2S: Why does my domain-specific Kirby config not kick in?

I am a simple man. A simple, forgetful man. This is part of the reason why I started this blog. Writing down things helps me remembering them—or gives me a place to look them up. So here’s another Note to Self (N2S) piece. Every now and then I find myself in the situation, that domain-specific configs do not take effect in a Kirby CMS setup. This shows most of the times in the debug option not…