Please ensure that the experiment "winter drop" is set on.Put sculk catalyst near the creaking.

If you kill the creakings created by heart,there will be nothing happening to the nearing sculk catalyst.No blocks will be turned into sculk.

If you kill the crekings created by spawn egg or command,you will see the nearing sculk catalyst bloom.But still no block will be turned into sculk.

I think when the creakings are killed,the sculk catalyst will bloom and the nearing blocks will be turned into sculk.