Hi all,
We had the same problem and we figured it out how to show back images. On MDL-36255 they added a format_string() function to the name of grade. This function strips all html tags if you have the formatstringstriptags setting enabled. If you disable this setting, you'll see it again in your gradebook.
Theme: Boost
Moodle version: Moodle 3.4.5+ (Build: 20180927)