Hi; many thanks oops66 for your quick answer.
oops66 wrote:Strange, I do not have this issue here (XnViewMP 0.78 x64 deb + Ubuntu 14.04LTS)
Hmm, too bad
oops66 wrote:be careful because near (not far) of this menu "EXIF Thumbnail - Rebuild" you also have "Clean..." and that can delete by inadvertence all meta-data.
PS: maybe you can post a .jpg sample here to a specific check.
You're right, but as you can see in attached screenshot (taken with my smartphone as with XnView, print screen feature does to work when menus are displayed

...), indeed I'm using the "reconstruction" menu line and not the cleaning one.
And, as shown in the same screenshot, once rotated a picture, when I want it to have its thumbnail reconstructed, I'm losing all its metadata.
Another screenshot is attached: is it possible that some thumbnail option is involved?