Issue upgrading Bootstrap with ui-select not showing multi select dropdown

I am getting an issue when upgrading Bootstrap css from version 3.1.1 to 5.2.0 where multi select dropdown stops appearing.

Example code on plnkr -

[https://plnkr.co/edit/9uJ9EsLb4Crtq801?preview]

It works well with Bootstrap 3.1.1 where dropdown appears nicely.

As soon as I remove link to Bootstrap 3.1.1 (line 28 in Index.html) and bring in line 29 which is bootstrap 5.2.0, the dropdown stops appearing.

  <hr>
  <h3>Simple String Tags (default theme)</h3>
  <h4>(With Custom Tag Label / Sort Enabled)</h4>
  <ui-select multiple tagging tagging-label="(custom 'new' label)" ng-model="multipleDemo.colors" theme="bootstrap" sortable="true" ng-disabled="disabled" style="width: 300px;" title="Choose a color">
    <ui-select-match placeholder="Select colors...">{{$item}}</ui-select-match>
    <ui-select-choices repeat="color in availableColors | filter:$select.search">
      {{color}}
    </ui-select-choices>
  </ui-select>
  <p>Selected: {{multipleDemo.colors}}</p>
  <hr>

Any help appreciated. Thanks