Affected version(s): all
Configured standard alerts settings for appointments in Apple macOS Calendar (Preferences → Alerts) for an OX App Suite CalDAV calendar account are reset to the default settings after the next sync with the server.
Apple uses an own extension of the iCalendar protocol to store the needed setting on the server. While there were efforts in the past to get these extensions standardized (Version 4 of the draft still contained some specification about this → VALARM Extensions for iCalendar draft-daboo-valarm-extensions-04), the related parts got removed from the next Version of the draft (→ VALARM Extensions for iCalendar draft-daboo-valarm-extensions-05).
Most probably the Apple calendar server is the only server which supports this functionality, all other CalDAV servers which are programmed according to the official iCalendar standards (as OX App Suite CalDAV extensions) do not support this.
As long as Apple macOS calendar makes use of these non-standard extensions for storing the settings there is no solution. Apple iOS calendar is not affected, apparently the settings in question are stored on the iOS device locally.