Just a tip / hint I came across this evening when updating the metadata to XMP files - if you have say for example 100 photos in a folder and several of them are 'stacked' - if you CMD/CTRL A to select all and CMD/CRTL S (or even if you do it from the Metadata Menu) to write out the metadata it will not write out metadata to the images that are stacked under the top image. You need to manually unstack them and select them to write out the meta data.
Not really a bug - but something to be aware of nonetheless as its easy to make the mistake of using CMD/CTRL A and thinking you have selected all the images in the folder.
