The issue with missing posts has been identified as an issue with the blockchain API. The devs are looking into it, but there is no fix yet. It does not appear to be related to using 5 tags though. You can track the issue here:
https://github.com/steemit/steemit.com/issues/986

A separate flag and downvote opinion is being discussed as well, but it would require a much larger change. If it were to be implemented, it would not be for quite some time, due to higher priority things.