In the past when I submitted an iOS App for the App Store, the reviewers would Metadata Reject the app if there was an issue with the keywords. You could then either try and convince them that it should be approved as is, or just remove the keyword and re-submit it for approval.
There have been reports this month (April 2013) that Apps have been getting approved (no rejections), but that keywords have been removed.
If Apple reviewers are now removing keywords from your apps without notifying you, I think this is cause for concern.
Let me know if you have come across this trend or not.