It seems there is a new bug in 5.7.0.0 of Garmin Express which prevents your PC from sleeping.
I noticed my PC was no longer going to sleep and tracked this down to Garmin Express:
$ powercfg /requests
DISPLAY:
None.
SYSTEM:
None.
AWAYMODE:
None.
EXECUTION:
[PROCESS] \Device\HarddiskVolume2\Program Files (x86)\Garmin\Device Interaction Service\GarminService.exe
Setting sleep prevention mode to ON.
PERFBOOST:
None.
ACTIVELOCKSCREEN:
None.
Right clicking on the Express Tray icon and exiting Garmin express solves the problem and it does not come back if you re-start Garmin Express until you do an update on a watch or other device. I guess express prevents the PC sleeping to prevent breaking a device during a firmware or map update, but it seems they forget to disable this again after the update is complete. :(
Hopefully someone will notice and fix this in the next update so I don't have to keep re-starting it.