Steps to reproduce the issue

Open a page with a page navigation (see buttons Prev - Next).
See also issue #20177.

Expected result

The tooltip of the page navigation is loaded with the bootstrap tooltip code.
See the image below.

Actual result

The tooltip of the page navigation is displayed as html.
Because the following script file is not loaded:
<script src=".../Joomla/media/jui/js/bootstrap.min.js?xxxxx"></script>

See image: bootstrap is not loaded

System information (as much as possible)

• Joomla! 3.9.0-alpha2-dev Development
• PHP 7.1.20

Additional comments

Possible solution:
Add JHtml::_('bootstrap.tooltip'); into the file ...\plugins\content\pagenavigation\tmpl\default.php

Image: bootstrap is loaded