Strange little bug. I'm running dual monitors and just needed to sync the timestamp on some images. I had the grid displayed on the 2nd monitor, filtered metadata for only 1 camera and then selected all.
I then chose Metadata > Edit Capture Time... from the menu but despite having 160 images selected the Change All button didn't appear, just change and sure enough it only changed the first selected image.
Now if I do the same but using the grid on the 1st monitor it works as expected and I can change all the images in one hit.
So it seems there's a bug that means multiple selections done on the 2nd monitor are ignored and only the first image is affected when editing the capture time.