Cheating is such a strong word. Some people think they can sign into a 'friend's account and help themselves to a few forge points that they put on the Great Buildings of themselves or others. This is breaking the rules. Everyone needs to let that sink in. You don't have to be given a label like 'cheater', just realize that it is surely, definitely breaking the rules to sign into ANY account except your one and only.
Another rule breaker that causes Great Buildings to disappear has nothing to do with dummy accounts. If I/you make a deal with any player to give my/your Great Building forge points in exchange for some favor on another world, that is breaking the rules. That often results in loss of Great Building.
When you refer back to posts made earlier, we don't know if any of this is the case so I am just putting information out there for readers to absorb. The opening poster needs to contact support in game to discuss his individual case.