Selecting the IFO file of the first Title, the menu thumbnail is correct.
Selecting the IFO of the second Title, both thumbnails are now from the second Title.
And it continues with subsequent Titles - the last Title selected becomes the thumbnail for all the Titles.
Note that it's the thumbnails that are wrong - the menu selections reflect the original Titles on playback.

If the Titles are indexed separately using DgIndex before importing (the .d2v file), the thumbnails are correct.

I select the IFO, else, no matter what, the video is reencoded even though it's compliant.