Hi @cje.churchill - From a Feature Implementation standpoint, this behavior is expected, although it may not be useful. It's also clearly not what you expected when working with Artboards. Arguably we should disable that command for Artboard documents.
Currently the command image > Image size will take ALL layers in the document and scale then, not just the selected layers. It's really intended to be used when working with whole document image data rather than object data. And you typically only do it once or twice. Design documents with Object layers like text, shapes and so on, have suboptimal results when scaled with Image Size. Yes, sometime the right things happen, but the results almost always do damage to the individual Layers as they are resized. Resize enough times, and the image quality will noticably degrade.
The better technique for resizing Artboard content is to convert the content to one or more Smart Objects, then scale the resulting Smart Objects as desired. This is non-destructive and will have superior results.