From the code you posted, are you distributing your app with the files within the .apk, instead of embedding?
In which case could something be deleting them, or just this one, file from the .apk. Obviously this should not happen, but there could be some badly written app, perhaps a virus scanner or free space recovery app, that is mistaking the file for something it should remove, perhaps due to its name matching some criteria.
Or it could be a malicious app such as a virus which has done damage trying to infect other apps. I don’t know of any such – I am speculating here. But if it’s only on one user’s device, then something odd and obscure might have happened on their device that is nothing to do with the functioning of your app.
One thing to try is get them to delete it and re-install it again. If that works then it suggests something damaged the file but a clean install fixes it.