I was fighting the ender dragon in creative with strength 255 for 1,000,000 seconds.  I had not eliminated the end crystals (and placed a few more).  I was punching it while it was flying, and I was flying.  I got the "free the end" advancement and it started doing the explosion thing that is part of the death animation but did not start glowing. I then killed it with commands, sorry.  I assume it would have given the experience, but kept flying.  I might also note that the duplicated ender dragon started with near to zero health.  I think this might be because I killed it on the same game tick that it gained health.

Steps

1. Go to the end
2. Don't remove end crystals
3. apply strength 255 for 1,000,000 seconds
4. go into creative
5. follow the ender dragon while spamming punching
6. Kill the ender dragon on the same tick it gains health
7. dragon should start making explosion particles, but remain in flight
8. Ender dragon duplicated