I’m using Maxxis flavour super light inner tubes/Ksyriums/Ultremo’s on the roadie. I’ve had a couple of failures round the base of the valve stem. Not riding along. It’s when it’s left overnight, I come back to it and find a flat tyre. There’s no obvious physical reason (sharp edges, lack of tape, valve cocked at an angle etc).

Any suggestion over why and how to avoid please?

No idea as to the “why” but I can recommend the new Ultremo ZX tubeless as a solution. Fitted some to my Ksyrium Elites a couple of weeks ago and they are great.