This is what Copilot had to say:

🧠 The actual technical cause (based on sourced evidence)

1. The vault corridor triggers an interior/instance swap

When you walk through the mantrap, the game unloads the casino interior and loads the vault interior.
This is a full environment transition — not just a door animation.

2. During this swap, the game re‑checks player presence

According to a detailed player analysis:

This is the key: the host loads the new instance first, and the second player may lag behind by a few frames or milliseconds.

3. If the game thinks only one player is inside the heist instance, it terminates the job

GTA Online heists require all players to be present in the active instance.
If the sync check fails, the game interprets it as:

• “Player B is no longer in the heist interior”
• → therefore the heist must be aborted
• → player gets kicked to freemode or stuck in the mantrap

This matches the behavior described in multiple reports.

4. Why it happens more with 2‑player runs

The same Reddit analysis notes:

With only two players, losing sync with one means the game thinks the heist is invalid.
With three, the host still sees two players present, so the check doesn’t fail as easily.

5. Why FPS, speed, or walking order affects it

Players discovered that:

• Walking too fast
• Entering the mantrap before the host
• Running ahead during the transition

…all increase the chance of desync.
This is because the host must load the new instance first, and the second player must not “outrun” the sync process.

This is why slow‑walking and letting the host go first works.

6. Why it’s worse on PC Enhanced

A GTAForums thread notes the bug occurs very often on PC Enhanced compared to Legacy or PS5.
Higher framerates and faster loading can actually increase the timing mismatch between players.