Dragon respawns in old world

When I load my pre-1.8 world in 15w31a and go to the end, the dragon respawns automatically! The console reports "Found that the dragon has not yet been killed in this world.", which is incorrect.

The problem seems to be that the game is searching for existing end portal blocks to detect "dragon was killed", but it only searches chunks south and east of 0,0. My existing portal is centered at -26,23 and so none of the portal blocks are found. If I make a copy of the portal at positive X and Z in 1.8 before loading the world in 15w31a, it correctly "Found that the dragon has been killed in this world already."