When grimhollow does his mortar attack you can hit him from behind to still do damage. When dealing damage to him this way the invincible blue block fx will still appear despite dealing damage to him.
Steps to Reproduce
1. Attack grimhollow from the front on his mortar attack, then from behind. Notice how the blue block fx remains.
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.