I had a similar issue. when I sent in a contact report of a task force Ii spotted. I got a response saying good find, and was issued another objective to "inflict heavy loses on enemy task force." This task force had 4-5 destroyers with two passenger liners. I sank both liners and this objective was still incomplete. I highly doubt I was supposed to also eliminate all the DDs - it would be a waste of torps.
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