A player summoned ender dragon will drop an egg if alive after a server reboot.

To reproduce:

1. Start a fresh end world
2. destroy all crystals
3. kill dragon
4. take egg
5. place 4 crystals around portal to overworked in end
6. wait for crystals and dragon to finish spawning
7. stop server
8. start server
9. see in console that the dragon has not yet been killed (see below)
10. destroy crystals
11. defeat dragon
12. find 2nd dragon egg above portal

seen in console after first dragon has been defeated, new dragon summoned and server rebooted:

[21:53:50 INFO]: Scanning for legacy world dragon fight...
[21:53:50 INFO]: Found that the dragon has not yet been killed in this world.
[21:53:50 INFO]: Found that there's a dragon still alive (EntityEnderDragon['Ender Dragon'/517, l='world_the_end', x=62.61, y=80.72, z=-7.31])
[21:53:50 INFO]: But we didn't have a portal, let's remove it.