Ender Dragon respawning doesn't work

Placing end crystals on the exit portal in the end does nothing, as seen on the attached screenshot.
This world was started in a snapshot, but quite some time after end crystal crafting was added.
I also hollowed out one of the obsidian pillars.

Edit: Found the issue. ExitPortalLocation in level.dat -> DimensionData -> 1 -> DragonFight was saved incorrectly. Y was saved as 56 when in fact 57 is correct. Not sure if that was only an issue in the snapshots or if it still persists.