Can't edit files on USB-Drive with iOS devices

Nextcloud version (eg, 20.0.5): 21.0.0
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): Apache 2
PHP version (eg, 7.4): 8.0.3

The issue you are facing:

I set up a Nextcloud 21 on my Raspberry Pi 400. To get more space and to safe some writing from the SD-Card I want to safe the files on a USB-Drive (ext4). For that I use the External Storages App in Nextcloud. That works well with the Nextcloud App, I can see and sync files to my MacBook and Windows Computers and also on iOS devices. However, if I want to edit my Markdown notes in any Markdown editor on my iOS devices I can’t see or edit the files on the USB-Drive, only those on the internal SD-Card. The automatic Photo Upload works fine to a folder not he USB-Drive. The USB Stick an the Folders on there have all the necessary rights (I think):

drwxrwxrwx 7 www-data www-data 4096 Mar 12 10:56 nextcloud/
drwxrwxrwx 7 www-data www-data 4096 Mar 12 09:27 Markdown/

Does anyone know what could be wrong?

Thanks for your help,
Stefan

I would recommend to check the content of the Nextcloud log file for any error message which might be related to the edit attempt.