I have run a short test and found that re-indexing simply adds the folder I renamed without removing the original folder. This leads to symptoms like question marks instead of thumbnails:
I renamed the folder “Test Set 03” to “Test Set 13” and back to its original name. Between each rename, I made DPL re-index the folder “Shared Stuff”, which is the parent folder. Re-indexing the parent folder mimicked the general re-indexing, which might have helped, if missing items had been removed, which did not happen in this test. Things like these make DPL’s database collect rubbish over time, rubbish, which cannot be removed, because the respective maintenance services do not exist.