I think you need to provide more information.
When you say "Lightroom modifies original image files", exactly what have you seen that is modified? In what way is it modified? Screen captures would be great, but more details are a must.
Thanks for the response. Following are some details.
In case of the original files being JPEGs or TIFFs, the file modification date and file size changes. In case of raw files (ARW in my case), a new XMP sidecar file is created or modified. The modifications seem to be limited to updating the timestamp (see the screenshot below). This seems to be a problem of Lightroom disregarding the "Automatically write changes into XMP" setting - when this setting is unchecked, Lightroom is supposed to write all changes only into the catalog and leave the original files intact (which is also how my Lightroom behaves in case of edits I do in the Develop module).
In the situation you show in your difference side-by-side, is the history record that is in the Before screenshot also in the XMP file in the After screenshot, perhaps down below, in other words are history records being added, or is the “saved” history record being overwritten?
The behavior your screenshot shows would occur if the files were used in the Camera Raw plug-in version of the Merge-to-HDR because ACR doesn’t have a LR database to write things into and always uses the image file to store the current state and to write the non-settings-related history records to. If this is truly a difference in LR 6.2.1 and hasn’t always been occurring, it seems like Adobe is using Camera Raw plug-in code instead of a LR copy that respects the Automatically-Write-XMP flag.
The history record is being overwritten, not added. The lines marked red in the screenshot are the only differences between the "before" and "after" XMP files.
I have not observed this behavior with LR versions prior to 2.6.1.
I just tried the "Photo Merge - Panorama" tool and found that it behaves in the same way, i.e., it modifies the original files, too.