The bug
Breaking blocks, that you can break with your hand, like weeping vines, twisting vines, flowers, sugar cane, end rods, torches, slime, etc., with shears reduces their durability. No other tools seem to be affected. This is inconsistent and weird. Most likely one will run into this while shearing tall grass and flowers.
Originally I only noticed weeping vines and twisting vines and breaking them with durability loss wouldn't be a problem, if you got a better drop chance, but there is no difference between using your hand and using shears, except that you lose durability. In this regard the new vines are special, that it could make sense for shears to take damage.
This may be an occasion to rethink fortune and silk touch on shears, since using them (legacy shears) also doesn't do anything to drop chance. And this would be their second use now (apart from breaking seagrass and coral fans).
Minimal fixed behavior: no damage to shears on blocks that you can break with no tool.
May relate to: MC-103430
Can confirm in 20w48a.