Bug is caused by "fix" of MC-8180
Grumm disabled putting silk touch on shears, but i think he forgot that silk touch on shears was intended and required to obtain cobwebs.

As wiki says:
Cobwebs can be obtained by mining them using shears or a sword[fn 1] enchanted with Silk Touch. It will drop 1 piece of string if the shears or sword are not enchanted Silk Touch, and will drop nothing when broken using anything else.
You are unable to put silk touch on sword, and since 15w47b you can't also put silk touch on shears.