Missing Mansions, I have created two different seeds and this has happened in both of them. No mansion at Map 1 location.

I set up all the infrastructure and traveled via the nether to villages, spent so many hours creating safe bridge walkways in the nether. to get to the location on Map 1. Map one of each different seed only to find there is no Mansion in the marked location. With the first one I actually found the image of the mansion online so I know was there for other players. I got rid of that seed and reloaded that seed. Then the mansion was half there. The back of the building was gone. I figured that seed was buggy so I created a totally new seed. I really like this would and spent a lot of time developing it as my forever would. Then set out for my first mansion. Its not at map ones location. In my very first realm would you added the mansions after we have been playing it a long time. We traveled to many of the mansion with no problems. Then all of a sudden the game became so lagged that we couldn't move. That why we started over. My friends have now all lost interest because its too much work for it to be broken. Why are the mansions not showing up for me. My current seed is 426999157851421,. I'm in a village near x=800, z=7300. The mansion is supposed to be north eat of me a little. The Realm world before this was -4589128118707775879. The mansion is supposed to be near spawn point. I don't know how to recreate it because I don't know why its happening. Its not like anyone can miss a Mansion. They are huge.