Could you please clarify what you mean by the FTP application in the UI?
To the best of my knowledge, our firmware has never included a built-in FTP application in the GL.iNet Admin Panel.
If you are referring to the fact that third-party plugins you installed are gone after a firmware upgrade, this is expected behavior.
When the firmware is updated, core system components and libraries are often changed. Automatically keeping third-party plugins could cause the system to crash, fail to boot, or experience severe stability issues due to version mismatches.
You can manually reinstall and reconfigure third-party plugins after the update is complete.