Ultra HDR is a new image format available with Android 14 on supported devices & camera. Basically it’s a JPEG image but some additional data added to it when captured on supported devices. With ultra HDR, images can have darker shadows and brighter highlights.
I thought the same thing at first, but it sounds like it is just that you can now view the “Ultra HDR”-ness within the app.
Also,
Google Messages doesn’t strip the gain map metadata from images