Hello.
The fix is to delete the submissions, not the strategies. Open the Submissions page and delete the relevant ones there, and they will drop out of the reports. For future, if the contest has already started, and they can no longer be deleted from your side, write us with their names or IDs, and we will remove them for you.
This behavior is by design, not a bug. A strategy and a submission are two different things. The strategy is the notebook in JupyterLab; a submission is a frozen copy of it, made at the moment you submit. The correlation checks, the leaderboard, and the correlation reports all reference submissions. Deleting a strategy only removes the notebook, which is what the "deleted" tab shows. The submissions created from it stay in the pool, which is why the leaderboard and the "correlation above 0.9" reports still point to them.