Ender Dragon Stuck

Whilst fighting the Ender Dragon, I found that it would get stuck in the un-activated portal. At first I thought maybe it was just the position of the portal, but it did the same thing in another world. To give more detail, the dragon swoops down towards the portal, and then seems to glitch in it, causing the dragon to stay still for a few seconds. I tried to get a screenshot, but it's hard to take a picture that tries to capture movement (or lack of in this case).