The bug
Nylium blocks generated in the nether are not randomly rotated like netherrack which creates an inconsistency in the how the textures are viewed and a parity issue with bedrock edition.
(This issue is not a duplicate of MC-184159 because that was only regarding the top texture of the block instead of the entire block.)
Linked issues
relates to 2
Comments 4
Should be marked as a clone of [MC-184159]
Relates to MCPE-105777
Can confirm, however this is most likely intentional. Very closely relates to MC-144914 and MC-184159.