Angular material table displaying empty rows

I'm trying to integrate Angular's material table in a new project but for some reason the data is not displayed, only the empty rows. I used the same logic in another project and there it works, I spent a lot of time to find the reason but nothing works.

I need to add the table in a lazy loaded module called "cores" and I imported the material table module:

import { MatTableModule } from '@angular/material';
imports: [
    CommonModule,
    CoresRoutingModule,
    MatTableModule
  ]

I created a services which returns an array of "ICore" objects:

getCores(): Observable<ICore[]> {
    return this.http.get<ICore[]>('/cores');
  }

The ICore interface:

export interface ICore {
  id: number;
  name: string;
}

The HTML from my "index.component.html" file:

<mat-table [dataSource]="dataSource">
  <ng-container matColumnDef="id">
    <mat-header-cell *matHeaderCellDef> Id </mat-header-cell>
    <mat-cell *matCellDef="let core"> {{ core.id }}</mat-cell>
  </ng-container>
  <ng-container matColumnDef="name">
    <mat-header-cell *matHeaderCellDef> Name </mat-header-cell>
    <mat-cell *matCellDef="let core"> {{ core.name }}</mat-cell>
  </ng-container>
  <mat-header-row *matHeaderRowDef="displayedColumns"></mat-header-row>
  <mat-row *matRowDef="let row; columns: displayedColumns;"></mat-row>
</mat-table>

The logic from "index.components.ts" file:

dataSource = new MatTableDataSource<ICore>();
columnsToDisplay = ['id', 'name'];

constructor(private coreService: CoreService) { }

  ngOnInit() {
    this.coreService.getCores().subscribe(
      res => {
        this.dataSource.data = res;
        console.log(this.dataSource);
      },
      err => console.log(err)
    );
  }

From my console.log I see that the dataSource is populated:

MatTableDataSource {_renderData: BehaviorSubject, _filter: BehaviorSubject, _internalPageChanges: Subject, _renderChangesSubscription: Subscriber, sortingDataAccessor: ƒ, …}
data: Array(3)
0: {id: 1, name: "core1"}
1: {id: 2, name: "core2"}
2: {id: 3, name: "core3"}

I also tried this method:

dataSource = new CoreDataSource(this.coreService);
columnsToDisplay = ['id', 'name'];

constructor(private coreService: CoreService) { }

export class CoreDataSource extends DataSource<any> {   
    constructor(private coreService: CoreService) {
        super();
    }

  connect(): Observable<ICore[]> {
    console.log(this.coreService.getCores());
    return this.coreService.getCores();   }

  disconnect() {} }

But with the same result. I feel that is something stupid that I forget to do but I don't find it.

Thanks in advance!