Is this intended?
If so, then it produces invalid HTML of nested <a href> tags in the many lists displayed this field, for example in my_view_page.php.

I'm not sure what the original intent was to be honest. But it does mess up the display in My View, View Issues, etc.
These's quite a few examples of such bad behavior on this tracker.

In my opinion, we should not allow any formatting (html, markdown) or replace links in the Summary - it should be plain text.

@atrol @vboctor what do you think ?

URLs in summary of Azure DevOps work items are plain text / links to the work item in work item list views and in the single work item view.
Issues on GitHub are treated the same way.
URLs in summary of GitLab issues are plain text / links to the issue in issues list views, but links to the entered URL in the single issue view (might be a bug)

In my opinion, we should not allow any formatting (html, markdown) or replace links in the Summary - it should be plain text.

So this seems to be the typical approach.
This approach could not be that good, if we ever should allow to create issues without entering a description (the description field is a hardcoded mandatory field)

Alternatively, we can leave this as is, as a feature and cleverly insert a link only up to the first occurrence of '<a ' in the formatted text.

Or drop the links, i.e. call string_attribute() instead of string_display_line_links() on the summary when it's displayed as a link ?

For the record:

• In my view page it used to be like that, the behavior changed with 0008565 - see my_view_inc.php line 344
• in view all page, the summary was not clickable until 0008066 - see columns_api.php print_column_summary() function

PR: https://github.com/mantisbt/mantisbt/pull/2124