The Guide Book says that the Devil's Snare grows in darkness, and spreads when fully grown- neither of which are currently happening.

The crop can be bonemealed, regardless of its light level- in fact, the crop never grows naturally (presumably because it's not bright enough?); it can only be grown with bonemeal/bonemeal-type items.

In bright light, the crop will indeed wilt again and revert some growth stages depending on the brightness, as described in the book. However, if planted in darkness (light level 5 or lower), the crop immediately uproots itself whenever it receives its random growth tick (tested & confirmed by increasing the randomTickSpeed gamerule).

The crop only appears to be stable at precisely light levels 6 or 7, according to my testing; any higher and it begins to wilt from brightness, any lower and it breaks due to being too dark. Furthermore, if two or more crops are planted directly adjacent, both will immediately break when either receives a random tick.

(Screenshot showing a diagonal cross of Devil's Snare, which is the only layout I can plant these seeds without them immediately popping back out again; the stable fully-grown crops are at light level 6, to the left you can see the crops have wilted more with increasing light levels, and to the right you can see any crops in further darkness have simply uprooted)

Versions
Minecraft: 1.12.2
Forge: 14.23.5.2847
Unique Crops: 1.12.2-0.2.6