Sea battle special task - what are the hidden conditions?

I picked Sea Battle as my special task. From the description, it should be very easy: achieve 2000 battle score in Naval AB. Use a tier 3 vehicle in battle. Repeat twice.

In practice, I’ve repeatedly had situations where I smash past the 2k battle score goal, yet the task doesn’t progress. What are the unmentioned limitations on the task? Do I need to finish the mission on a rank 3 vehicle?

Somehow missed I need to win… back to the drawing board.