Yes I looked a bit more and you might be mostly right. Basically the readme.txt should be embedded in the game binary, but it seems that when built to release, the file is not being included inside. This might be a configuration error on my side, because there are inclusion and exclusion rules, and I’m using both.
In debug, the file exists outside of the game, so it is capable of loading it as a file separate from the game like you said :P.