No, the banks don't, but I have had that happen to me with other financial services companies' portfolio management and custody platforms and systems.
In some cases, the balances are zero and all the holdings vanished. They come back at the end of the maintenance window, but it's bl**dy disconcerting the first (and second) times it happens.
Simply several strands of inadequate management coming together to p*ss the customer off. Sadly, the norm. . .