and it works. This directive must be set on global section
The following options must be set in the global smb.conf section and won't take effect when set per share.
But need to check if there won't be an issue with TM share because bigSur wants to set some permissions when it creates a TM backup...
---edit---
a TM backup is now running. all seems to be ok but need to wait complete and try another one after disconnect/reconnect the TM disk to be sure