It's not a bug, as mi10 lite 5g has a notification led xiaomi decide to not activate always on feature for more than 10 seconds. Stupid, but not a bug. But if is for you necessary to have it, there is a solution. Go to settings, additional settings, developper options, at the end of the list, desactivate miui optimization. The always on option will stay '10 seconds' but in fact aod will work correctly.