Nextcloud is only backing up one of many calendars?

I host my own NextCloudPi instance and use the Nextcloud app on Android. I have different calendars I synchronize via Nextcloud and DAVx5 on my smartphone, to have them on my desktop and laptop as well. I also use the apps feature to create automatic backups of my calendar. But for some reason it’s only backing up one of my four calendars, even though all of them get synchronized over all three devices (there are only “calendar1.ics” in the folder “.Calendar-Backup”.)
Can anybody help me get all my calendars to be backed up?
I’m using NextCloudPi v1.46.7; Nextcloud v22.2.2.0; the Nextcloud Android app v3.19.0 and DAVx5 v4.1.1-ose.