void ApplyStateOnLastHandler()
{
    handlersExecutedCounter++;
    if (handlersExecutedCounter == eventsToPersist.Count)
    {
        // Using creates a COPY of nextState with stateData set to nextData
        // nextState still refers to the OLD version WITHOUT the NEW nextData being set
        base.ApplyState(nextState.Using(nextData));

        // uses OLD nextState, but that doesn't matter, as Timeout hasn't changed        
        CurrentStateTimeout = nextState.Timeout;

        // HERE's the issue and the actual bug
        // The "AfterTransitionDo" callback (ie, AndThen) - if present - is
        // called with the StateData Property of the OLD, UNCHANGED state.
        // That is, this StateData is always lagging one version behind.        
        nextState.AfterTransitionDo?.Invoke(nextState.StateData);

        // the rest is not relevant
        if (doSnapshot)
        {
            Log.Info($"Saving snapshot, sequence number [{SnapshotSequenceNr}]");
            SaveStateSnapshot();
        }
    }
}

When(LookingAround.Instance, (evt, state) =>
{
    // NB: When in this state, the cart is empty
    if (evt.FsmEvent is AddItem addItem)
    {
        // visitor adds an item to the EMPTY cart
        return GoTo(Shopping.Instance)
            .Applying(new ItemAdded(addItem.Item))
            .ForMax(TimeSpan.FromSeconds(1))
            .AndThen(cart => /* what to you expect cart to be? Empty or non-empty? */);
    }
    [...]
});