Solved.
Aha. This was not about putting the computer to sleep, but merely turning off the screen, which had me deeply perplexed. How could that affect the software?
Well it turns out that I have USB ports on the side of my Dell UltraSharp monitor, very convenient. I had the USB e-licenser in there. It never occurred to me that of course when the screen is off the USB power is off also, so the e-licencer powers down. Then there must be a side case where when the power is reapplied the communications between Dorico and the USB dongle is not re-established, with disastrous consequences.
Moving the dongle to a USB port on the computer not subject to the screen power of course fixes the problem.
So, although there is an issue here, but I understand it may be a limitation of the USB protocol and not a fault in the USB dongle design, the solution is bleedingly obvious, once realised. It does not matter if this problem gets fixed, as it’s just not the correct place to put the dongle. Very stupid of me in hindsight.
Issue can be closed.