This is because mat-select does not allow toggling multiple after the component has been initialized.To solve this, you must destroy the original field, create a copy with the modified multiple value, and re-insert it at the same index. After that, trigger Formly to re-render using this.fields = [...this.fields].

1. this.fields = [...this.fields]; is critical to trigger Formly’s internal change detection. 2)this.cdr.detectChanges(); is optional but recommended in some cases (especially inside async logic