Share mounted via mount.cifs: rw but read-only

On a new Samba-4.22 file server I have 2 shares which point to the same 
tree of folders:

[xy]
	path = /mnt/data/smb/xy
	read only = no
	hide unreadable = yes
	
[xy-mailbox]
	path = /mnt/data/smb/xy/Scans/mailbox
	browseable = no
	read only = no


I edited the ACLs via RSAT to achieve this:

a group "xy-users" should have change rights on [xy]

a user called "some-reader" should have rw-rights on [xy-mailbox]

The customer wants to use the share [xy-mailbox] in a specific debian-VM 
and writes a systemd-mount-unit like:

[Mount]
What=//192.168.150.24/xy-mailbox
Where=/mnt/mailbox_xy
Type=cifs
Options=credentials=/etc/samba/mailbox_xy_credentials,vers=3.0,uid=988,gid=982,file_mode=0660,dir_mode=0770,_netdev,cache=strict

There are several such mount-units, using separate users for mounting 
multiple shares from the same samba-server 192.168.150.24.

the mounts work and are shown as "rw"

But I can't create files/dirs within that mount.

I am unsure how to approach such "nested shares". Getting the ACLs 
correct always confuses me.

Is there anything specific I can check first?

I will mimic that mount on my local system to cross-check things.



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