0.1K is excellent, so any COP issues would seem to be real (unless theres an issue with the flow rate sensor which is the second measurement entering here).
If you’re a bit technically inclined you can get an ebus adapter to log all those internal measurements. Not as accurate as a full heat metering setup but cheaper and will give you a great start. There’s a couple of threads on that here as well, e.g. Vaillant eBUS hardware adapter (ebusd software) Thread