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Steps to reproduce the issue

Use any template or otherwise incorporate Bootstrap into Joomla.
Enable the User - Profile plugin.
Change "Terms of Service" to "Required"
On "Select TOS Article", choose an article.
Attempt to register a new User via the frontend.
"Terms of Service" link will not be visible, only the "*" for required fields.

Expected result

This line in the source uses a "modal" class on the link.

<a href="/index.php/component/content/article/12-club-docs/2-constitution-and-bylaws?tmpl=component&amp;Itemid=101" class="modal" rel="{handler: 'iframe', size: {x:800, y:500}}">Terms of Service</a>

Bootstrap contains its own definition for .modal:
.modal { position:fixed; top:0; right:0; bottom:0; left:0; z-index:1050; display:none; overflow:hidden; outline:0 }

Actual result

The line only renders a "*" with no link to the complete terms and conditions.

System information (as much as possible)

Joomla 3.9.1
Helix Ultimate Template v 1.0.5
Bootstrap v4.1.3

Additional comments

Source of class definition is in /plugins/user/profile/field/tos.php:
line 84:
$attribs['class'] = 'modal';