i would like to add that it seems like the speed of the convoy has a big impact on the behavior. when the movement speed is low, the route in the test mission will complete completely fine.

after doing even more testing, it seems that the followers in the convoy "freeze" most often on intersections and very sharp corners.

Confirm. Sometimes even the commander's vehicle freezes.
I can even say that after updating to version 2.16, sometimes convoys ignore the added waypoints.
A workaround for resuming following the leader's vehicle:

_groupLeader = leader _group;
_groupLeaderVehicle = objectParent _groupLeader;
_groupLeaderVehicleSpeed = abs (speed _groupLeaderVehicle);

{
    if (((abs (speed _x)) < 5) and { (behaviour (effectiveCommander _x)) != "COMBAT" }
        and { _groupLeaderVehicleSpeed >= 5 }) then {
        _x doFollow _groupLeader;
    };
} forEach (_vehicles - [_groupLeaderVehicle]);

@Schatten i appreciate your contribution. however, i want to make it very clear that i wish that this issue gets resolved by a developer from Bohemia Interactive. Because that is what needs to happen in order to make regular players enjoy the already impressive AI convoy behavior. It just has flaws that ruin the whole thing.

@Nelis75733126, I would also like this bug to be fixed. Until this bug is fixed, you can either use my workaround or wait who knows how long. The choice is yours. :)