The PSU is working correctly and the rails are within spec. There are numerous threads over on the Keysight forum where people have had the same loss of sync when the backup battery dies, so it's clearly some NVRAM settings that get corrupted and set the display frequencies incorrectly. The cure is to reset the config to default, but without the 8590B service manual, I don't know the key sequence; annoyingly, a couple of people have said that they fixed the problem, but didn't explain how they'd done it and neither of them appear to be using the support forum regularly and haven't replied to my messages.