I’ve seen it mentioned anecdotally online a few times, and asked Vaillant about it at this year’s Installer Show… they confirmed it, but didn’t have an engineer on their booth who knew the specifics though.
I’ve only had this happen twice, when our entire 250-litre tank was fully depleted in the middle of winter, and likely completely filled with <10C mains water.
@Andre_K I’m pretty sure that there is no mechanism to use the DHW store as a defrost energy source. I’ve read plenty of accounts of Arotherms ending up in defrost death-spirals due to lack of space-heating water volume, without ever dipping into the DHW store.