I was on a survival/creative singleplayer world. i'd already had a couple nether portal crashes so i gave myself blaze rods. i went to the nearest stronghold and went inside, it looked completely untouched. no other entrances, doors already open, etc, but when i found the portal room it was already complete and the silverfish spawner was not there.
i was confused but went in, mined up only to see the dragon was already dead, the end was lit up fully with torches all over and the end was completed