HP 15c Collector's Edition save/restore does not work

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Shortly after posting here, I found that the issue is really caused by low battery voltage. While I just bought the HP15C CE, both batteries where at ~2.7V. Obviously enough to work in principle but not enough for a reliable USB mode. I replaced the batteries and now the HP15C CE is detected at once in USB mode.

Of course, this raises some questions regarding the HW design.
Is the calculator drawing so much current in OFF mode that it depleted the batteries after just a year? From what I can tell, this is not the case for an original HP16C at least. Then again, the HP 35s also drains batteries quickly in OFF mode.
Also, the low battery indicator doesn't seem to work at all and there is not even a brown-out detection to reset the device, instead it just freezes.
These kind of design flaws are not exactly what you would expect from a HP calculator.