Well my calendar has just sent me a 24hr notification before an event only problem is the even was last week not this week!!!
I encountered the same problem like yours for my P6000 alarm clock this morning, 6:15am one when off just fine, but 6:30am one did not and saw a missed notification when I woke up at 6:45am.
Trying to figure out what the problem is myself, but BLACKVIEW ADMIN, please look into your software and fix the problems.
I replied to your other post but basically as i already rooted my P10000 i removed the clock app and installed the google version which is the same app just better version and till the problem was there so it isnt the app itself its got something to do with the background activities. even though i have whitelisted everything it still has the problem.
It seems to me that Blackview (and other Chinese manufacturers) include all these background apps so it makes the phone look like the battery is performing better but then it screws with the actual workings of the phone.
My notifications, email, alarm, calendar, youtube etc are so unreliable iv switched back to my Ulefone Power 3 until they issue an update which will fix alot of these issues. Infact updating to Oreo 8.1 as promised may help but it doesn't seem like they want to!