It seems that the support of a long path support is incomplete in FileSeek (Version 6.7).
Windows used to have some limitations regarding the length of a file path (MAX_PATH = 260 characters) and length of the length of path to a directory (260-12=248 characters). The limits are described in https://docs.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=cmd. This still applies to the command line but e.g., using the Windows explorer, it is possible to exceed these old limitations.
If you create a file which exceeds the path limit of 260 characters using the Windows explorer you are able to search for the file using FileSeek and you will find your test file.
However, some functions can no longer be used or even will silently fail:

• Select your search result and choose “Copy selected file”
  FileSeek will open an error prompt with the message "Windows does not support this action with long path names.". If you search for the file in the Windows explorer, it is possible to copy the file.
  
• Select your search result and choose "Zip selected result"
  FileSeek will not display any error message and create a zip file at you desired location. However, if you open your zip file, you will see that the selected file is missing. In my case only the parent directory structure of the file has been created. Using other applications e.g. 7z it is possible to create a zip archive with the selected file and the folder structure.