I have a recurring problem where calender events are created with no reminder. I have tried to set the default reminder option in settings, but the app keeps forgetting it and goes back to no reminder (which is a stupid default option to use for a calendar). I’ve tried to set the default option at least three times and it works for a while, but as soon as I forget to carefully check each new event’s reminder setting, it tends to reset back to no reminder.
Setting it that way applies to all calendar folders. The option can also be set individually by right-click on the particular calendar folder and choosing Properties.
In some instances, if the server forces a complete resync, the properties for a folder will be reset. But it may be something else. Set it as you described, so on all folders, and when it reverts to no reminder, check the calendars from other servers and Local Folders calendars and see if it is reset there also.