What version are you using.
My file files.lst contains filename swith the extension .cpp.
D:\Dev\Root\Common>ffind /tcpp @files.lst
0 Zeilen in 0 Dateien
Help Index "@File List"
A file list is simply a standard text file containing the names of the files to process, one per line. This allows you to create a list of files for processing using output from DIR /B, DIR /F, or FFIND, a text editor, or any other method that produces a file in the proper format. Both absolute and relative paths may be included in the file, However, wildcards are ignored, and each line is processed literally, without any further checking. This means that if a command allows options to restrict operations based on age (/U, /C), ranges (/I..., /[d..., /[t...), attributes (/A:), or location (/S), those restrictions will be ignored when processing the @file contents.
I use TCC 22.00.41 x64 Windows 10 [Version 10.0.17134].
The help says that You can CREATE a file list with ffind, NOT that You can use a file list as input to ffind.
In fact, the help lists commands that can read a @file list, and ffind is not there.
Also, if in Your file list You have files with ".cpp" extension, the ffind command that You use here would try to find the "cpp" text INSIDE the files, not in their names.