Hello.
I noticed a mistake for me when the dungeons leading to the End portal appeared. Twice it happened to me that this object was just at the bottom of the ocean, once just below the surface of the earth. I tried to find a portal on Creative mode several times, but then the portal was under Mega Taiga Hills, under a lake in a Birch Forest. I do not know if this problem has occurred yet. I am enclosing the screens of the above-mentioned cases.