This is indeed a really strange issue. As you describe it, it really looks like to me that you accidentally activate the "sticky keys". In order to check if this is the case, please disable sticky keys from windows settings and try if it happens again.
Also this doesn't seem to be a campaign or mission bug, since all missions where designed and implemented in the same way and didn't occur during testing to none of the testers or my self.
Thanks for sharing this issue, later in the day i will try to reproduce it and post the results.