Angular validation messages appears after reset() form

It looks like you are using Angular Materials. If so ,you must reset FormGroupDirective too,only resetting the FormGroup is not enough.

private registerForm(fData: any,formDirective: FormGroupDirective): void {
    formDirective.resetForm();
    this.RegForm.reset();
}

<form [formGroup]="RegForm" #formDirective="ngForm" 
   (ngSubmit)="registerForm(RegForm,formDirective)">