In the image attachment template (image.php) there must be something wrong with the following lines
<div class="nav-previous"><?php previous_image_link( false, esc_html__( '<i class="fa fa-chevron-left"></i> Previous', 'sparkling' ) ); ?></div>
<div class="nav-next"><?php next_image_link( false, esc_html__( 'Next <i class="fa fa-chevron-right"></i>', 'sparkling' ) ); ?></div>
Instead of Next/Previous buttons you get buttons that say “<i class=“fa fa-chevron-left”></i> Previous” and “Next <i class=“fa fa-chevron-right”></i>”