Currently if you summon a new dragon in a END world if that dragon is killed this not generate a new end gateway or the exit portal (if was destroyed). in teory this issue is because custom dragons not has a dragon fight instance where this things are handled.
Steps:
Join to a end world
if has a normal dragon kill with commands (or use a END with already killed dragon)
Summon a dragon (summon /summon minecraft:ender_dragon)
Kill that dragon normally
End Gateway and exit are not generated
Not a bug, only summoning-ritual spawned dragons result in fight-related changes, and it has been that way since 1.9 when the fight got revamped.