I am using version 2.9.7.
My site uses MediaBox popups for some images so that the user can click on a thumbnail to see the full size image. Existing popups work correctly. However, in the current version of JCE, if I modify a popup or create a new one, clicking on the thumbnail displays a white box as expected but then, instead of expanding and displaying the image, it collapses to a small vertical oblong containing the caption (although most of that is invisible as the box is too small for the text) but no image. Having compared working popups with non-working popups, I have found that JCE is now adding the parameter
data-mediabox-type="image/jpeg"
to these popups. Removing that parameter results in the popup functioning correctly.