It appears that leaving the Battle.net app open as the active app/window with World of Warcraft selected under the games tab will not allow MacOS to go sleep.
I found this after repeat mornings of coming back to the computer after playing WoW and having left this specific tab open and finding that the computer was awake when I returned hours later.
To verify the issue, I tested this by setting my Mac to sleep after 1 minute. I clicked on each tab News/Shop/Social/Games and then the Games individual tabs. Only when selecting World of Warcraft under the games tab will my computer not sleep. All other tabs will let the computer sleep. The issue is still present if I turn off the “Use browser hardware acceleration” option under App settings.
I originally noticed this issue under Catalina but did my testing using the latest Big Sur (11.1).