Well I'm afraid I don't know why it's behaving like that but I'd like to say two things:

1 Welcome to the .org

2
In the Imperial Campaign folder the file is missing
The Imperial Campaign takes half it's files from the Base folder (a couple of levels up). That folder contains the default campaign files. If a file doesn't exist in a campaign folder, it gets it from the Base folder.

If you ever want to edit the Imperial Campaign, just copy the descr_strat.txt from the Base folder to the Imperial Campaign folder and edit away.