disk_free_space in combination with btrfs

Dear php developers, I am using nextcloud which is developed on the base of php. My system: Open Suse Leap 15.2 PHP 7.4 Filesystem: BTRFS (configured as raid1) As far as I can see, nextcloud uses a php build-in function called "@disk_free_space" (https://github.com/nextcloud/server/blob/41d95d9e5a8b4fe0ee1ad8f80243bde29a61ac50/lib/private/Files/Storage/Local.php#L372). Unfortunately, on my filesystem, the returned value is wrong. I can't find anything regarding this function and BTRFS except for a statement from some guy on github, who refers to the php documentation. I had a look there as well but I did not find this note anymore, which he refers to. (Other conversation on github: https://github.com/osclass/Osclass/issues/533#issuecomment-10716859: "we should not rely on this function on Linux BTRFS filesystems".) Is this function ment to work on BTRFS or is it a known bug? Is it possible to implement a fix? I guess this fix should be done directly in php instead of in the nextcloud code (check filesystem and use different method to get the free space...) Kind regards Georg