That's the biofilm barrier, and here's why it's so hard to get rid of.
Your toothbrush can't break through it. It only scrubs the surface. Your tongue scraper only removes the top layer. Even flossing can't penetrate the sticky matrix between your teeth where the bacteria hide.
And these protected bacteria? They produce sulfur gas as a byproduct. That's the rotten-egg smell and metallic taste you're experiencing.
Each time you brush or scrape, you might clear some surface bacteria. But underneath, in those tiny grooves on your tongue and the gaps between your teeth, the bacteria are safe and protected by that sticky layer.
Within hours, the biofilm rebuilds. And the cycle starts over.
The bacteria multiply. The biofilm thickens, and the sulfur production increases.
That's why your breath smells fine for 30 minutes after brushing, but then comes right back.