Hello,
I have had this happened a couple of times, although most of the time I don't have this problem, and I haven't been able to locate the option to stop this.
Simply put, when I select the Page Tool in the toolbox and I resize the page, once I release the mouse button the page will go back to the previous size. The only way to change it has been by inputting the size manually.
I am using Web Intent in the document properties, if it makes any difference.
Thanks in advance!