As mentioned HERE a simple File > Rename of file should probably not suggest the directory specified with Default Open/Save file Directory when that setting is not Follow current document. Rather, it should always suggest the directory that the file is currently in. Optionally, a new preference setting could be made for controlling this functionality.

Debug Information

Notepad++ v8.1 (64-bit)
Build time : Jun 17 2021 - 01:56:07
Path : C:\............\npp.8.1.portable.x64\notepad++.exe
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 1909
OS Build : 18363.1556
Current ANSI codepage : 1252
Plugins : mimeTools.dll NppConverter.dll NppExport.dll