ECS1250 Slim Desktop will not wake from sleep to run scheduled tasks

I recently acquired this desktop as a new Win 11 machine, and installed my backup software, which allows me to create a schedule.  The schedule can be configured to wake the machine if it is in sleep mode.

It quickly became obvious that the backups were not running when the machine was asleep.  They would kick in soon after I returned, or after a reboot, but the wake timers are just not doing anything.

I've eliminated the backup software as a cause, by creating a simple scheduled task to run a batch command file.  This just echos the current date and time to a text file.  When I get back to the machine I can look at this file to see when the task ran (if at all).

The task just doesn't run when the machine is asleep.  Something in the machine's config is preventing the wake timer from doing the waking that it should do.

I've been through all the usual checks in Power Options - wake timers are enabled.  The sleep mode is S3, so it's nothing to do with the modern S0 sleep mode.  I even turned off Fast Start, but that led to a disastrous situation where the machine would not boot.  I have at least got it back again after a system reset.

So - what experience do you other users have?  Is this a known issue with Dell machines, or with the ECS1250; and is there a solution?