Are you using the stock clock app on your device?
-If yes, do you experience these symptoms if you test your device in safe mode?
If your phone is misbehaving, it's possible that a third-party app is causing the issue. To figure out what app is acting funny, you can use the Safe Mode feature. In Safe Mode, your phone won't run any third-party apps. This allows you to easily identity and remove the app that may be causing the problem. Give this link a try:
Have you tried force stopping and clearing the cache and data for the clock app on your device?
- Go to settings > apps > clock > tap on force stop > then tap on storage and clear the cache and data
I would recommend wiping the cache partition on your device. The system cache partition stores temporary system data. It's supposed to allow the system to access apps more quickly and efficiently, but sometimes things get cluttered and outdated, so a periodic cache clearing can help make the system run more smoothly.
Please keep me posted if you continue to experience these symptoms.