That’s a known issue. Currently you have to use a workaround like shown in your question.
This is working as intended. When the change event is emitted ngModelChange (the (...) part of [(ngModel)] hasn’t updated the bound model yet:
<input type="checkbox" (ngModelChange)="myModel=$event" [ngModel]="mymodel">
See also
- https://github.com/angular/angular/issues/3406,
- https://github.com/angular/angular/issues/6311