I am guessing here, but with my limited knowledge, it could be a hangover from being purely ICE.
With a full EV like the Enyaq, the big 400v battery will have an inverter that will supply charge to the 12v battery, which is required for the stereo, aircon, other ancillaries and will possibly allow a charge to the 12v when plugged in.
With the Octavia and Superb iV being hybrid, I would guess they still have an alternator to provide power to the 12v, and if you run the car in E Mode without the ICE running, it will never charge the 12v as it doesn't have the inverter like a full EV.
Maybe the trick is to at least once a week, drive it like you stole it to make sure the ICE kicks in and the alternator spins up to charge the battery.
Interesting to know for when my vRS iV arrives in a few months.