It's because you took them to the surface. They wilted.

Confirmed on 8/8/23

10/08/2023: For me it didnt work. Put the Flowers in a "Bewildering Adventures Backpack" and that Bag into a "Backpack". As soon as i teleported to the blighted Village all my 3 flowers turned into wilted petals.
Did some further testing:
- Putting it in the Clothing Cest didn't work as well as the normal Backpack.
- Putting it in the "Chest of the Mundane" will let you keep the Flower as it will turn into an ordinary object. But the Flower will lose its Anti-Magic Propertise, even if you take it out of the Chest again. So will only get a useless flower.
Well guess here goes my Anti-Magic-Barb :((

Thought the same as a Duergar was "casting" Invisibility. Checked the Combat Log later and as it turned out it was a Class Action. Class Actions don't count as spells.
Might be the same case.

Worked to block Filro casting when I fought him with Karlach holding the Sussur Flowers.

same thing here, i wonder if there is any other way to find sussur bloom again...

Well, nowhere. Unless the Gnome sells them in the myconid colony, I dont remember.
Explored everything in underdark finally, and found them only on two places...Just loot it next time and sell or send to camp :D

In your next playthrough

I wilthered in the camp chest either way, even though i did not leave the underdark. i guess camp still counts as surface either way

Cuz the chest itself is magic

You can just loot the mundane items and they'll turn into their real form - don't need the flower

Druid received an inspiration when using the flower. Not sure if you still get this if you just loot the items.

The flower does not transform the items in the chest. The chest itself turns the items inside into mundane objects. Bringing the flower to the chest just blocks the chests magic. But as other have pointed out you can just take objects out of the chest and they transform back.

Understandable mistake but yeah chest of mundane is a special chest that turns any item placed within to look like common items such as cutlery. So the items found within said chest you saw were always there all you had to do was take out the cutlery and they'd turn back into their original form.

Granted it is interesting that the sussar bloom also nullifies the effect the chest of the mundane has which is very well done by the developers.