Here are some different ways to reproduce. All use attached "18156_formation_leadership.Stratis" repro mission. During this mission a hint will inform you if you are group formation leader or not.
METHOD 1
1. Preview mission.
2. Notice nearby AI board a quadbike. He will drive close to the player, and pause for 10 seconds. Board the quadbike in cargo position. Notice hint says player is not group formation leader.
3. He will then take you down the runway, away from your subordinate units.
4. Observe your subordinates ordering you to return to formation.
5. Wait for the quadbike to stop, then get off. Notice hint says player is group formation leader again.
METHOD 2A (may affect people used to right hand drive vehicles more than others)
1. Preview mission.
2. Notice nearby sports hatchback. Get into it as cargo (player loses formation leadership).
3. Move into drivers seat and drive away. Notice your AI ordering you to return.
METHOD 2B
Follow steps in 2A, but use the Hunter instead of the hatchback. Notice no bug!
METHOD 3
1. Preview mission.
2. Notice nearby sports hatchback. Order unit 2 to get in as driver. Once he is in, board as cargo (player loses formation leadership).
3. Order unit 2 to drive you to the other end of the runway.
4. Before you arrive at the destination, order unit 2 to stop.
5. Order unit 2 to get out (player is now apparently formation leader again).
6. Get out. Try to order your units to regroup, but your unit will say "all join group - 2".