Object not added to one $scope variable but to all others - angularjs

Question

Everytime I want to add a form to fill out a new contact person of a customer company I call this function:
$scope.addContact = () => {
$scope.selectedCustomer.contacts.push({});
console.log(JSON.stringify($scope.selectedCustomer));
};
But I noticed there is one customer that magically loses that contact object at somepoint after the method is called and I don't know why, because in the debugger only some standard angularjs methods are executed that shouldn't play a role in this and I don't understand them.
Usually the console looks like this:
1st, 2nd and 3rd click on add contact button results in:
{"id":135,"masterData":{},"name":"Normal Test","contacts":[{}]}
{"id":135,"masterData":{},"name":"Normal Test","contacts":[{"$$hashKey":"object:2205"},{}]}
{"id":135,"masterData":{},"name":"Normal Test","contacts":[{"$$hashKey":"object:2205"},{"$$hashKey":"object:2207"},{}]}
But for one object (so far, but I think there are more) the same situation prints this:
{"id":150,"masterData":{},"name":"Test AG","contacts":[{}]}
{"id":150,"masterData":{},"name":"Test AG","contacts":[{}]}
{"id":150,"masterData":{},"name":"Test AG","contacts":[{}]}
As you see there is an empty object added but not saved, every time I click add contact again, the contacts array is empty at first.
In the mongo db those two customers look exactly the same:
What else can I analyze to find out where the problem is and what to do about it?
EDIT
By debugging I found the place where the workflow traverses a different path when it's not working:
contentscript.js, line 1114 var observerHandler = function(mutations) {... is traversed when it works, but not reached during the failing process - it is called after: return typeof jQuery !== "undefined" && jQuery.event.triggered !== e.type ? jQuery.event.dispatch.apply( elem, arguments ) : undefined; in jquery.js, line 5013.
My HTML:
<div class="form-group">
<label for="contacts" class="col-sm-3 control-label">Ansprechpartner</label>
<div class="col-sm-9">
<table id="contacts" ng-if="selectedCustomer.contacts.length"
class="table-condensed" cellpadding="10">
<tr>
<th>Name</th>
<th>Telefon</th>
<th>E-Mail</th>
</tr>
<tr ng-repeat="contact in selectedCustomer.contacts">
<td>
<a class="delete" ng-hide="readonly" ng-click="deleteContact(contact)">
<i class="fa fa-minus-circle"></i>
</a>
<input type="text" class="form-control"
ng-readonly="readonly" ng-model="contact.name"
placeholder="keine Angabe">
</td>
<td>
<input type="text" class="form-control" ng-readonly="readonly"
ng-model="contact.phone" placeholder="keine Angabe"/>
</td>
<td>
<input type="text" class="form-control" ng-hide="readonly"
ng-model="contact.email" placeholder="keine Angabe"/>
<a href="mailto:{{contact.email}}" ng-show="readonly">
<input type="text" class="form-control"
ng-readonly="readonly" ng-model="contact.email"
placeholder="keine Angabe"/>
</a>
</td>
</tr>
</table>
<div class="addContact" ng-hide="readonly">
<button class="btn btn-default btn-block btn-sm" ng-click="addContact()"
type="button">+ Ansprechpartner hinzufügen...
</button>
</div>
</div>
</div>