this is not a bug in the software. I went back and did everything you explained and my "new" subtitles over the old ones remained and were visible on my DVD after it finished burning.
As for your comment "what might be ok for one group of friends . . . ." I totally agree------the cool thing is this is already part of PhotoDVD.
You can have up to 3 different types of subtitles (for example one in english, one in french, one in spanish---or description, date, funny comments)
The best way to learn more about this is to take a quick peak at the manual http://download2.vso-software.fr/manual/PhotoDVD2_manual.pdf (full of screenshots)
otherwise, the basics are to select subtitle track 1 (to click on, found on left side of text box where you enter subtitle comments) start at beginning of all you pictures and go through all of them writing those "genre/types" de comments (for example subtitle 1 funny comments)
then go back to beginning of picture select subtitle 2 and go through all those picture writing for example your 'descriptive' comments
and so forth
you do not need to put a comment on every picture nor do you need to you add all three tracks of subtitles
Hope this helps