I have the Lenovo Moto Z Unlocked with sadly still Android M.
There's a hidden alarm everyday at 12 midnight, and it doesn't show it in the original clock app, which I haven't used an app other than that for alarms.
How can I diagnose this problem?
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Sign up to join this communityI found the problem. It turns out TimeTune was doing this (even though it is only a reminder app). I uninstalled and it disappeared.
Check for Google Calendar or any other task app that you have installed. Possibly you might have setup notification as daily and time might have be automatically selected as 12 PM.
Do remember that Factory Reset is the final resort to solve your issues, including this one as well.
I did everything that anyone in any sites suggested but it didn't solve, then I uninstalled clock app updates,restart the phone,and the hidden alarm disappeared!