ordering of selected items unexpectedly changes when using a custom filter

[shusson]

We are finding that the order of the selected items (only in the UI not the ngModel) is being affected by the results from a custom filter.

Example template

<ng-select
    [items]="search$ | async"
    [typeahead]="autocompleteInput$"
    [multiple]="true"
    [clearSearchOnAdd]="true"
    [closeOnSelect]="false"
    [(ngModel)]="filter.values">

    <ng-template ng-option-tmp let-item="item" let-searchTerm="searchTerm">
        <span>{{ item }}</span>
    </ng-template>

</ng-select>

Reproducbile example
https://github.com/shusson/test-ng-select

To Reproduce

1. setup up and run angular app from https://github.com/shusson/test-ng-select
2. click on ng-select component
3. add all terms
4. search for d
5. add all terms
6. search for d
7. clear search term (delete d)

Expected behaviour
The order of the selected items doesn't change.

Actual behaviour
The order of the selected changes. It seems to be based on what is in the results of the filter. The bound filter.values doesn't change ordering.

Screenshots

Desktop (please complete the following information):

• macOS 10.14
• Chrome 69.0.3497.100