You're using layers without realizing it, plain and simple. Happened to me all the time - ALL THE TIME - until I started using the object manager docker window. If you draw in certain spots on the page or outside of the page layout, Corel will often start a new layer on your behalf (no idea why).
Look very carefully and you will likely see that some of your objects are on master guides, master page, or master grid, or master desktop. To corect this in a file you're working on, zoom out and select everything. Cut everything onto the clipboard, then repaste it without doing anything. All objects will now be on the same layer again.