I just experienced this on a multiplayer Arbitration mission on Mithra. The first time it showed the "inactivity" thing was as the "reward" for Round 2 at the end of Round 3. I thought it had happened because I waited at Point A while someone killed the last enemy at Point C at the end of Round 2, so the game had thought I was AFK, but at the end of Round 4, the rewards showed the "inactivity" thing for Rounds 1 and 3, so I knew it was a bug.