All I can really tell you based on mine is that when I turn on the automatic clock-set feature, when I return the next morning (radio is ‘off’ all night) the clock is WAY off. Either a station here is sending bad data, or the radio is misinterpreting what is sent.
I haven’t tried to mess up the clock by hand, then turn on auto-set, then sit and watch to see what happens.
If we can't get RDS clock time to play nicely with HD reception, maybe they could allow RDS clock updating to work in standby mode. On the other hand, I think Sangean has also addressed the clock problems in newer HD radios. My HDR-16 and HDR-14 clocks are sufficiently accurate to leave them in manual mode.
I suddenly don't think we have quite the same issue. Standby mode (what I previously called power off) is _when_ mine updates itself to something goofy. My -18's clock is very accurate so long as I set it by hand and leave synchronization turned off. It has stayed exactly the same as time.gov since I've set it to match by hand.