Gravatar not being correctly displayed

Recently, I have decided to change the default gravatar. Upon changing, none of the avatars in the comment section of the post have changed, but only those in the comments in admin panel.

I initially thought it was a cache-issue, until someone posted another comment, with its default gravatar to be the same as the initial one.

I begin to suspect it’s a theme problem. However, my get_avatar code seems to be correct:

<?php echo get_avatar($comment,$size='48',$default='<path_to_url>' ); ?>

Any ideas?