Killing ender dragon no longer opens end gateways

I have killed the ender dragon and spawned 2 end gateways before. However, killing the ender dragon no longer generates end gateways. I looked under the hood and found that the field in the level.dat file called Data:DragonFight:Gateways keeps getting reset to having all 20 entries (indicating that all the gateways are closed). When I kill the dragon this does in fact take the last entry out the list (leaving 19) but like I said, it keeps getting reset to all 20 (perhaps when I open the world and or enter the End but I am not sure). It should be noted that this world was not generated or has ever been in 20w20b or 20w21a