I've been doing a lot of multiplayer TTF runs lately, using Individual drop style, on both Viridia and Whitill section IDs. In all of the 100 or so runs I've done over the past week or two, the rare drops from both Vol Opt and Dark Falz have never dropped a single time for any member of the party. The rare drops for the Dragon and De Rol Le bosses, on the other hand, have dropped many times. Either we all have really terrible luck or there is a bug preventing these items from dropping.
At least two times when Falz died, I heard the rare drop chime, but no one in the party actually got the item. And I've never heard the rare drop chime for Vol Opt, period.
*edit*
Actually now that I think more about it, I'm not sure if the Dragon boss was actually dropping his rare or not. I heard the rare drop chime several times for it and assumed it was dropping but I was teleporting to the Caves asap so I can't actually confirm. The Del Rol Le boss was definitely dropping his rare because I picked it up myself a few times.