Angular app stops changing text on button click after user input

You have to use ngModel instead:

<textarea id="txt1" [(ngModel)]="txt1"></textarea>

Make sure your main module also imports CommonModule:

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }