I thought it worth sharing here that Apple (in the UK) admitted to me that the battery on iPad Pro 12.9 could indeed decrease even when plugged into the supplied 12W charger, if in use at the time. I thus argued it was not adequate, and to my surprise they sent me a 29W charger for free. They even gave me a USB-C lightning cable when I said I had no way to plug in the new charger. Anyway, thought it worth passing on as I read many blog posts about this issue where people paid out for one. If anyone here needs proof to support their own claim feel free to message me.