There's speculation going around on what types of coffers these drop from. I've had people say they only get it from gold/purple coffers.
Today I managed to drop a spearhead cloth scrap from a green reward coffer, so I can for sure confirm they drop from any of them, although the higher quality ones might have better RNG for it, in case anyone was interested.
I do not think they have a cooldown, as I've gotten two heavily played cards one after the other, so a lot of it is probably RNG whether it drops or not.
Hopefully this is helpful to those farming the fragments. ^^
@Djennku, PCNA.
Grand Master crafter, all styles and all furnishing plans known pre U41.
Vamp and WW bites available for players.
Shoot me an in-game mail if you need anything, happy to help!