Hi, PBIStaran.
I'm glad you decided to reach out on the Apple Support Communities forums today. I understand that the second alert settings aren’t saving after you delete them and/or completely recreate the event.
Are you able to change the default alert setting in your Calendar app? You may need to change the default to get the settings to save when you’re deleting the second alert.
Change default alert settings in Calendar on Mac
I would also make sure that you’re following the guidance in this section of the Calendar User Guide for managing your event alerts:
Set event alerts and receive notifications in Calendar on Mac
Best Regards.