It didn't sound like a problem to me (before you involved dealer, that is
), but without more details, it's hard to know. Do you know how much they topped out last time? And how many miles are you talking about when the indicator went down to 1/2? By the way, at least my '08 goes from low to half to full (you also have overfilled and more than 1L low). To understand the system, figure there's a probe at full, at half, and at low. When level goes below full, it'll show half, but that doesn't mean you're half low. It'll continue to show half until going below half, at which point would show low. Am I making sense? It's best to also understand how to read level correctly (do a search), and do the top-off yourself, so you also know how much oil your engine is consuming (if at all) over time.
Oh, and the crappy stock filler cap needs the gasket coated with oil (like if it was a canister oil filter), or it could bunch up and make a mess, which is probably what happened to yours. Yeah, it shouldn't be like that, but techs should know better. Good luck man.