If it is per design the application needs write access - perhaps setting permissions for the applicaitons installation path to the system user created by the PKGBUILD.
→ PKGBUILD - aur.git - AUR Package Repositories
This is a guess - I don’t know the application in question - but I do think the application is designed around various tunnelling protocols used to circumvent country based network restrictions.
I suggest you create an issue with the maintainer of this package - as it appears to be a packaging issue.
Another option is to provide a full path /home/$USER/singbox/data/<config> instead of the relateive path data/<config>