Hi @LightCy I think that you should put the phone in Safe mode and verify the behavior of Always on Display. If the problem does not persist in safe mode, you will have to uninstall apps to verify which one causing it.
I will also mention some things that can help you. You can also make sure that all apps are updated in the Playstore and Galaxy Store. After you can you go into device care and run the optimisation tool and clear the ram too. If a particular app is affected, then go into settings, apps, choose the app, storage, clear cache and if it is really necessary clear data. As a very last resort back up and factory reset then set the phone up as a new phone.
