This is probably because in the bbpress plugin there is a template called content-search.php. Many themes also now have a template file called content-search.php. Since the bbPress templates don’t necessarily need to be in a bbPress folder, the bbPress plugin is choosing the template from your theme before the template that is actually in bbPress.
To fix copy content-search.php from the bbPress plugin templates and place it in a child theme in a folder called bbpress.
Functions files and child themes – explained !
So create a directory on your child theme called ‘bbpress’
where %your-theme-name% is the name of your theme
Make a copy of this file, and put in in the directory called bbpress that you created above, so you end up with
bbPress will now use this template instead of the original