I believe you. I've been checking it. I've tried to reproduce the problem and I've asked people who have complained about it before to do the same.
With that, I must say that "Please check it, it's a bug" makes me a little upset. It's mostly due to my own failure to reproduce the problem. I encourage you to file a bug rather than complain about issues on the forum. That gives us a better place to talk about it than a thread like this where lots of people are giving input. Also, you said the issue started occurring a couple of weeks ago? Did you mention it then?
For relevant log files, let's start with the standard ones at Contribute to UBports - Bug Reporting. Please follow the instructions there to file a bug. Some more relevant information... Is the device plugged in when the alarm is missed? What is your "missed alarm time" set to in Clock settings? Does the notification you missed the alarm appear after that time? Does the alarm always fail to ring, or only sometimes? Do you use your device heavily before the alarm fails to ring? There's a lot of information that is potentially relevant that we just don't have here.
I understand this issue is upsetting to you. I'm sorry I haven't been able to reproduce it. Please help me by following the normal process to file a bug report so we can get more opinions.