There are no necessary rights on the mounted disk when using samba and transmission packages

Actually I don't know why you are making it so complicated, you always change transmission folder? Even it's just 1 single command to change owner of folder from shell, I don't think this is too much for you, right? And you don't even need to figure out how to change the setting of Samba. Unless you are going to have many instance of transmission?

How about you close the "bug" on GitHub, too ?

User error <> actual error.

I too have had these issues. I use the CIFS packge to mount a cifs volume on my nas. It is possible to use the uid and gid options with cifs to set permissions on mount. In my case neither those options or chown will change the permissions. However that is not an issue with openwrt, in my case the nas I am using does not support changing permissions. I also use the noperm option which I thought might get around it but the seccomp and ujail settings in the init.d for transmission still look for the correct permissions. My solution was to comment out the seccomp and ujail lines in the init.d. This has been previously documented in the transmission github. Whilst not so secure, it is the only option that worked for me. I blame my nas for the issue..