When you export your game, you have several possibilities to define what files should be exported.
By default, only resources (i.e. files that Godot knows are necessary since they are loaded by scenes) are exported, which means that if you load a text or config file from code, it won't be exported.
You can fix that by selecting the "Export all files" option, or by whitelisting the specific files or file types you want to include in the data pack.