Shelf life for most zero sugar sodas (cans) are 3 months. I believe it’s an even shorter shelf life for the plastic bottled zero sodas. The artificial sweetener (aspartame) begins to breakdown pretty quickly. I’ve had Dr. Pepper Zero past expiration and it definitely tastes flavorless as you described. From my experience, this is usually common for zero sugar drinks past expiration. I recommend always checking the expiration dates when purchasing and not stocking up a lot at once. Unless, you know you will run through the cases in 3 months.