Task is added to the wrong place of the ordered tree when store is sorted

[bmblb]

Steps to reproduce:

1. Load this dataset to Gantt:

tasksData : [
    {
        id       : 1,
        name     : 'Task A',
        duration : 1,
        myfield  : 4
    },
    {
        id       : 2,
        name     : 'Task B',
        duration : 1,
        myfield  : 2
    },
    {
        id       : 3,
        name     : 'Task C',
        duration : 1,
        myfield  : 1
    },
    {
        id       : 4,
        name     : 'Task D',
        duration : 1,
        myfield  : 5
    },
    {
        id       : 5,
        name     : 'Task E',
        duration : 1,
        myfield  : 3
    }
]

2. Sort by myfield ascending
3. Add task below Task E

Related issue: #7220