There’s a lot standing between your app and what it can edit and save: POSIX permissions, ACLs, SIP, TCC, and maybe the sandbox too.
Have you found Time Machine docs helpful in solving problems? Do you know what “custom access” means in the Permissions section of Get Info?
macOS now adds special permissions to standard folders including your Home folder, and its special folders inside it. This explains those ACLs and how to fix them.
Permissions, ACLs, SIP, TCC privacy protection, and now DataVaults – Mojave has many ways of stopping software from opening files and folders.
Setting permissions in Terminal is simple. This basic introduction explains how to understand the notation used, and save yourself time fiddling in the Finder.
When copying files across in Target mode, their permissions are left in a mess. Why?
How did some of my old disks and folders get to have ‘custom access’, and how can I fix that?
Using TinkerTool System or MacPilot to make easier use of the Shared folder using ACLs.
Disk Utility warns of SUID files and ACLs. How can I complete repairing permissions?
You can use ACLs to ensure that all files created in a shared folder inherit the permissions of their parent folder.