Check the app showing next alarm, even if none is set
I have this problem on Android 8.1.0 (Miui 10.2 by Xiaomi) The time of the next alarm is also weird Initially it was 00:00, later it was some random time in the afternoon
I am also experiencing this issue, on MIUI 10.3.4. No alarms set in Simple Clock or stock Mi Clock, and no events in the calendar. But both the Simple Clock app and widget show an alarm for midnight (but this has shown different times in the past, e.g. some random time in the afternoon).

seems to work well in https://f-droid.org/en/packages/me.jfenn.alarmio/ and https://f-droid.org/en/packages/com.better.alarm/ according to a reporter
or nevermind, those 2 apps dont even show scheduled system alarms anywhere...
App shows a random alarm time even though no alarms are active or set. If an alarm is set, then it shows up as expected at the first screen. Tested on Android 11 (Samsung A32 5G)
if anyone is still around, can you check if the issue isnt related to some other apps notification? Like a calendar should show a reminder at 9:00 so Simple Clock shows 9:00 as the next alarm?