The main battery, from what I was told, does keep the 12v topped up. However, the 12v batteries that the VAG fit (like other manufacturers nowadays) are, putting it nicely, “not a premium product”. From what I’ve read those that have had batteries replaced from non-VAg sources like the RAC etc haven’t had further issues.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!