Mike,
With two images and maybe more on one print page nested, with Optimal first and then to Center the smaller image does not center vertically but sticks to the upper side and with Optimal Spaced to the underside. The same when I start with Center. The first image (Tiff) was nested with Original Sized, the second image (JPEG) specified on the longest dimension fits just vertically in the print page. In the past they would both center vertically and center/distribute horizontally. No Photo Mats used in this case but I think this is related to the new features as I has the same experience some days ago and can not recall that it ever happened in the past.
If I give them the same vertical size they both will center and I can switch several nesting options and both behave as expected. Giving one the Longest Dimension fit and the issue reappears.
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Met vriendelijke groet, Ernst
http://www.pigment-print.com/spectralplots/spectrumviz_1.htmApril 2014, 600+ inkjet media white spectral plots.