Snow golem wither rose farm

A wither rose farm built on a small platform in the End. You summon a wither into a sealed one block pocket, then feed snow golems to it: the wither's projectiles kill them, and a mob killed by the wither effect drops a wither rose. Two dispensers place carved pumpkins onto snow blocks to build the golems, an observer and a lever run the cycle, and the roses collect below. The redstone is nine items and takes minutes to build. The real cost is the wither skulls and the steady supply of snow, which is why most builders pair it with a snow golem farm.

Place the wither's tail on the zero zero block, not its body centre. The video does it one block off and the wither breaks out, which is what HuntZanetti and williampaine3520 both traced. Several people lost hardcore worlds to this.
Set the difficulty to Normal or Hard. On Easy the wither effect never applies and nothing drops.
The farm stopped working from 1.21.6. niborss, Pekotero and chinesefood3165 all point at the projectile target tolerance change, and the symptom is that the snow golems simply never die.
gaminggranny confirms the same failure on 1.21.8 and craznate2347 on 1.21.11, where it still breaks snow on the ground and produces nothing else.
Zestlera downgraded a world from 1.21.8 to 1.21.4 to build it and got 2,004 roses out of it.
notnow7302 points out the snow supply is trivial: two snow blocks and a pumpkin, trap the golem in a one by one space and it makes snow forever.