subprocess.Popen calls _winapi.CreateProcess. The current_directory is a wchar_t* string (Py_UNICODE*) passed to Windows CreateProcessW() (Unicode flavor of the API).

https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file#maximum-path-length-limitation

"In the Windows API (with some exceptions discussed in the following paragraphs), the maximum length for a path is MAX_PATH, which is defined as 260 characters."

I guess that the workaround is to use the "\\?\" prefix for "extended-length path". For example, "\\?\D:\very long path".

=> "The Windows API has many functions that also have Unicode versions to permit an extended-length path for a maximum total path length of 32,767 characters."