Is it possible that you have multifactor turned on for Nextcloud?
If so, you can create a one-time password in your NC settings.
User 1 creates a calendar and shares it with user 2. There are several appointments in this calendar. Each appointment has a reminder 1 day before.
User 1 has an Android phone and uses the Davx5 app and it will display 1 day’s reminder
User 2 also uses an Android phone with the Davx5 app. It will not show any reminders.
In the web browser, are the same problem
Either the calendar is not fully shared with reminders, or the Davx5 app can not read anything or is not assigned by nextcloud.
If I give the user 2 a write permission to test and only open the appointment and update without changing anything, the 1-day reminder will also be displayed on the Android user 2.
Enclosed screenshots from Google Calendar once from user 1 and user 2. Both have the same calendar from nextcloud.
With Apple the same problem could be determined
When the same problem also happens in the Web browser and with Apple, it’s probably Nextcloud related. Did you already ask there?