I can't be sure where the modification is occuring.
When I move/copy a DNG file to my macos device (the one I code on), the image does not appear to be modified. All the exif and xmp metadata appears intact.
I then copy the image into my simulator and when I run OAiOS, and load the image, debugging statements tell me much of the metadata is stripped.
When I copy the image to my ipad, and then run OAiOS, and load the image, debugging statements tell me much of the metadata is stripped.
I'm thinking that under the hood, the iOS libraries are stripping much of the metadata. What I think it is happening, is that the libraries are first converting DNG to JPG and then loading. That conversion code is probably only transferring/copying a subset of the metadata not on purpose but because its only coded to xfer some subset.
This theory sorta jives with the ImageMagick convert utility. On linux, when I use convert to convert DNG to JPG, the exif.focallength* params seem to get dropped or not carried over to the JPG.
Fun with iOS.
Bobby