changing document size
I am using the latest Version of Indesign CC 2018. (13.0).
The following bug exists in the previous version as well.
Create a new indesign document of (for example) 500 x 500mm.
Next, place some single objects close to the border, but stay inside the documentformat.
Now, go to "File; Document settings", where you can adjust the document size. Decrease the size from 500 x 500 to (for example) 400 x 400, with the Preview-Button ENABLED.
Wait for the moment until the Objects move OUTSIDE the document border, because of the to smaller fileformat.
Now, again increase the the document size to 500 x 500.
All Objects, that are OUTSIDE, remain there, and don't move back within the document format.
As confirmed by the user, the issue is resolved.