Based on Herald rules, there will occasionally be a team like Security added to a review as a Project Reviewer.
If a non-security reviewer accepts a diff, but nobody from Security accepts it, then it cannot be landed, which is correct behavior.

However, if the reviews happen in the reverse order, with the Security reviewer accepting first, then the diff can be landed even though it hasn't been reviewed by the requested reviewers. This is a problem because it is taken out of the queues for the requested reviewers, which makes it harder for them to find it.