Test plans are where QA time goes to die: hours transcribing a feature spec into cases, permutations, and edge conditions — clerical work that must be done carefully and is therefore done slowly. When the sprint is tight, the plan gets thinner exactly when the feature is riskiest.
And plans written by one person have one person's blind spots. The reviewer who would catch the missing negative-path cases rarely exists; QA reviews everyone else's work and nobody reviews QA's.