What kinds of UPnP settings?
As I know, it can be disabled or enabled and there is a Current UPnP Settings List.
Do you mean the list goes blank af ter you reboot the router?
If you press Refresh button, does it make a difference?

Yes the list goes blank after reboot, did not try refresh, but my server and other things weren't accessible from outside ( that is how I noticed)

Do you mean the UPnP entries are cleared?

UPNP are not static entries. Of course they will be cleared when the router is rebooted.

But the host should be able to generate new entries a while after reboot.

Upnp entries were cleared but only after rebooting when I change wifi channel !
I notice that after few hours when I want to access my home server from workplace. I have Synology server and I have to follow the procedure Router configuration/set up router.