There seems to be some unusual behavior happening here. When the multiple property is not included, the select function works properly. However, when the multiple property is added, the options disappear. Feel free to test this out for yourself by clicking on the following link.
https://stackblitz.com/edit/angular-yezybn?file=app%2Fapp.component.ts
<select multiple>
<optgroup label="Properties">
<option *ngFor="let eachVal of notObjProp" value="{{eachVal.property}}">
<div>{{eachVal.property}}</div>
</option>
</optgroup>
<optgroup label="References to Properties">
<option *ngFor="let eachVal of objProp">
<div>{{eachVal.property}}</div>
</option>
</optgroup>
</select>