Network connection disables at sleep

August 2020 iMac with a brand new (December 2021) logic board (replaced under warranty)

The desktop goes to sleep at the appropriate time (controlled by Energy Saver settings -- about 15 mins). Wake for network access is checked. Enable Power Nap is checked.

When I come back to the desktop and push a key to get to the login screen, upon log-on, there is no network connectivity. I have to wait a good 30 or more seconds for connectivity to restore. It happens faster if I unplug the ethernet and plug back in. Otherwise it'll reset by itself (always) after 30 or so seconds.

Is there any way to disable this behavior, such that the desktop never turns off its network connection during sleep?

The iMac is not using wireless. It is hard-wired by cat6 to a switch, router, and 1 Gbps fiber optic connection in the attic.