Ember bootstrap form input character prevention

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	
	@tracked
	username;

	@action
	onFormSubmit(){
    
  }
	
	@action
	onUsernameChange(newUsername){
    if(newUsername.length > 0 && 'a' === newUsername.charAt(newUsername.length - 1)){
    	newUsername = newUsername.substring(0, newUsername.length - 1);
    }
   	this.username = newUsername; 
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
<BsForm @formLayout="vertical" class="user" @model={{this}} @onSubmit={{this.onFormSubmit}} as |form|>
	<form.element class="d-flex align-items-center" @required={{true}}  @controlType="text" @property='username' as |el|>
  	<el.control @placeholder="username" @onChange={{this.onUsernameChange}} class="form-control-user"/>
</form.element>
<BsButton @type="primary" type="submit" class="btn-user btn-block" data-test-submit-btn="true">Submit</BsButton>
</BsForm>
<br>
<br>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-bootstrap": "3.1.3"
  }
}