Thanks for the suggestions murali, I found the issue in customer.first_name argument.

    <mat-form-field appearance="fill" class="customer-dropdown">
    <mat-label>Select Customer</mat-label>
    <mat-select (selectionChange)="onCustomerChange($event.value)">
      <mat-option *ngFor="let customer of customers$ | async as customers" [value]="customer.id">
        {{ customer.first_name }}
      </mat-option>
    </mat-select>
  </mat-form-field>