To conclude:
The basic issue here is that there are some clients out there, that support HTML markups in descriptions of tasks and calendar items (like e.g. Thunderbird, Em-Client, Outlook) and others, that don’t (like e.g. various mobile apps or the nextcloud calendar/ task app).
This leads to a compatibility issue once HTML capable clients exchange descriptions in HTML format.
Now I found out that the standard solution for this situation seems to be, that clients that do not support HTML markups delete the HTML representation once the description is updated in these clients - and only update the text only field “description”. (see Calendar ignores ical DESCRIPTION;ALTREP when updating event description · Issue #3863 · nextcloud/calendar · GitHub)
In this case, the sync chain via Nextcloud with Em-Client stays consistent. Cause Em-Client just also clears X-ALT-DESC (as it is no longer provided in the iCalendar item received from Nextcloud) and updates the (text only) description field with the new data.
Therefore I asked the developer of my favorite Android calendar app (CalenGoo) to change it accordingly. This has already been implemented and now it works fine for me again.
I also asked the Nextcloud developer to change it likewise in the Nextcloud calendar app and in the tasks app for X-ALT-DESC. The fix for the calendar app has already been provided. And for the tasks app i just requested it (Fix: Inconsistencies due to ALTREP parameter by max65482 · Pull Request #1916 · nextcloud/tasks · GitHub).
So at least I am (finally) fine now.
@Master_DJon, @Nitaro:
If you still face this issue, you might want to ask the developers of your other calendar apps to implement the same behavior?