SteSto86

Solution:
You have to change the UMASK for external storage in the config.php of the nextcloud docker. The file is located in the nextcloud appdata share.
Go to the Shares tab on your server and navigate to appdata->"nextcloud"->www->nextcloud->config. The first "nextcloud" should be replaced by the name of the share you created for nextcloud.
Open the config.php and add following line: 'localstorage.umask' => 0002. The standard value here is 0022, which doesn't work for me.
Restart the docker container.
 
This thread pointed me in the right direction, special thanks to Dmitr1: https://help.nextcloud.com/t/external-storage-files-dirs-created-with-wrong-permissions/68969/23
 
I saw that this changes were integrated June 2022, so you need a recent update. Versions 24.0.9 or 25.0.3 should work. I have 25.0.4.
 
I tested with files/folders created through WebUI and Android app and the fix works for me.