When mining crimson or warped nylium without a silk touch pick, the block will dropped as netherrack. You have to mine nylium with silk touch in order to get the block.

How to reproduce:
1. Get two picks, one with silk touch, and one without.
2. Mine both types of nylium with both pick types
3. You will see only silktouch drops the nylium blocks

This behavior does not match 1.16 java snapshots, where you do not need silk touch to aquire nylium.

Possibly related to MCPE-67367 ?