I have had this problem for a while and I really can't figure out what is causing it. I currently work in Windows but have had the same issue on a mac.
My document is at A3 size with facing pages. On my master pages I have a few objects; a logo, a footer text and a page number on each page. My master page is set up as a spread with a mirrored design, but I also get the same problem if I split the master into a right page master and a left page master.
When adding a new spread (2 pages) to the document nothing happens to the master layout, everything is fine. But if I only add one page at the time all my master page items are displaced and slide out to the sides, almost as if the added pages are larger than A3. This happens on all the pages in the document AFTER the added page.
I have tried to lock the objects on the master page and untick 'master page overrides', but this does nothing. I only have A3 size pages in the document and can't understand why adding pages would change the size and layout of the master.
Can anyone explain this? It has been bugging me for ages. Please let me know if more info is needed.
If anyone has the answer I will be forever grateful.
On a facing pages file that is asymmetrical, when you override a master object (by placing content into a masterframe, for example) it loses its link to the position, so the coordinates remain the same and it wll appear to shift if the page cahnges sides. Add or emove pages in pairs to avoid this.