[Lizard]

In your system, a completed flagged task has no usefulness. But flags are fairly flexible. Someone else might be using them in an entirely different way. Perhaps reviewing the things they did last week and flagging everything they did that cost money, to help them get control of their budget. In this scenario, it would be strange to them to not see completed flagged tasks when they filter for flagged tasks.

So I agree that one should be able to filter down to "flagged and not yet completed" tasks, but I don't think that should be the behavior of the "flagged" filter, as it overly restricts that feature.