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.