"Accept Invitations" Question: Understanding Import of Event Reminders

I’m using NextCloud 13.0.6 (or similar, where can I look up NextCloud’s version number if I’m no admin user?) and Thunderbird 60.2.1 and am having real trouble with event reminders for event invitations I get…

Most of the time event reminders contained in received invitations (also sent by another user via this nextcloud instance, event also created using Thunderbird) are just ignored when I accept the events in Thunderbird.

The received invitation mail contains the reminder just fine:

DESCRIPTION:Mozilla Standardbeschreibung

The accepted event in the Calendar does not have any reminder set.

Why is it getting lost, who’s responsible for this (Thunderbird or Nextcloud?) and is there any way to fix this?

Bump… Is noone else having any problems with calendar reminders?