Hmm, that is unlikely. Did you by any chance move one or more of your layers? If any of your layers have an offset (you can check this in the Layer Properties), then this would cause the exported image to go beyond the map boundaries. Currently it is relatively easy to offset a layer by accident…
Ah… yes. I thought I was already checking for that, but there was an Image layer I’d forgotten about that had a significant offset. (Is it possible to move an image around -without- affecting that offset? I’d like to be able to place that Image Layer wherever I’d like, because I use it as a template it helping me design, but -not- impact the exported image.)
Yeah, if you create a “Collection of Images” tileset, and then add your reference image to that tileset, then you can place it as a Tile Object on an Object Layer. I believe the objects do not affect the size of the exported image.
Doing it that way allows not only positioning but also rotating and scaling the image if necessary. If you need to avoid accidentally selecting the potentially large background tile object, you can lock its layer.