It has been a long known issue, I’ve been running into that since I have been using eM Client, starting with version 7.
I think that thread from 2016 might be the same issue
It also has been discussed here
Yet, the proposed solution is only a bad workaround. I want to use the Nextcloud feature “Birthday Calendar” together with eM Client, just without annoyance.
Maybe eM Client pokes the Nextcloud people directly to find a solution together?