This is weird, I'm not entirely sure what I've done wrong, but when the game moves from one layout to the next the tiles from the first layout are kept and then more tiles are added. I've got screenshots from the first two layouts of my game prototype and a screenshot of the second level of the game, where you can see the tiles from the first layout are still at the bottom as well as the new tiles for level 2.
Here's the image gallery to explain what I mean:
Make sure the tiles are not set to global in the object properties. Otherwise the problem might be in your events if you are creating the tiles manually.
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Yes! The tiles were set to global. That was the problem. Thanks so much for your help.