When instantiating an object from prefab by this way (in an empty project, Unity 2020.3.2f1):
myObject = Instantiate(preObject, Parent.transform);
This one changes myObject's shape very much. Actually, I don't know why.
Found a decision:
myObject = Instantiate(preObject);
myObject.transform.parent =Parent.transform
Is this a bug or Im just that lazy, I can't read documentation?