I’m not entirely sure if permissions features were completely disabled through FTP. I know they were disabled in PHP and in the file manager.
The important thing to understand is that we run PHP code as your account user. That means that your PHP code is always able to write to all files and directories (within the htdocs folder of your website). You do not need to change any directory permissions to make that work.
Only if the software was built poorly. 755 permissions are sufficient for websites to write their own code. Only if the software specifically checks that the permissions are set to exactly 775 (due to a misguided sense that this is necessary) would this cause problems.
To be clear, BookStack only thinks it can’t write to those directories, because it actually can.