In this case you should combine both implementations. Instead of setting the form data and using the … operator use this.form0.form.patchValue( { PropertyName: TargetValue } );
OLD:
this.artikel = { ...this.artikel, Nummer: event.value, NummerTeil1und2: event.value + ' ' + this.artikel.Nummerteil2 }
NEW:
this.form0.form.patchValue({ NummerTeil1und2: event.value + ' ' + this.artikel.Nummerteil2 })