"Free the End" advancement is not granted when killing the dragon with a bed explosion

What I expected to happen was...:
When I killed the ender dragon I should have got the "Free the End" advancement.
 
What actually happened was...:
The ender dragon died normally but I didn't get any advancement for killing it.
 
Steps to Reproduce:
1. Go through the end portal.
2. Kill the ender dragon using a bed.