Hi Laura
Many thanks for taking the time to reply, and apologies for the delay in responding. I have done a bit of testing and now have some more information that may help you to identify a fix.
First, my files are JPGs, from a Panasonic TZ40 at max quality, so files are about 7Mb - too big for some of the uses that I put them to.
I find that the 'original' file, as copied from the camera to my PC, is absolutely fine - i.e. is 'presented' in portrait form, as it was taken with the camera.
However....two strange behaviours:
1. If I rotate then save the original file from portrait to landscape, it looks OK on screen [i.e. it is correctly rotated as per my actions]. But if I exit the file, the thumbnail is NOT rotated and re-opening the file gives the un-rotated version. [i.e. my rotation actions have not been saved].
2. If I copy the file to another folder, it stays in portrait format. If I then re-size the file [25%, H 918, W 1224] and save, the saved file is presented as landscape and, as above, attempts to rotate it back to portrait do not succeed.
I think you are on the right lines when you surmise that the 'rotation' tags are being lost/altered.
Hope that helps.
David