You will need to keep the copy of the file on "C:\Hakros Doom Launcher 1.6\data\games\heretic\" to use it with the app anyway.
All iwad files are copied to engine folder each time but normally it works fine, it is estrange that only that file fails.
At the moment I have no solution I will try to see, if I can reproduce the error in some way.
Greetings, and sorry for the inconvenience.