transparency problem

[flashtt]

I'm creating a resoucepack with models , like stairs. When in vanilla Minecraft, it works well. 

 

when Minecraft 1.12 with only forge mod ( version 1.12-14.21.1.2415 ) 

 

I try some Minecraft versions (1.8.9/1.9.4/1.10/1.11/1.12)，same results.

only when I used 1.8.8 with forge （1.8.8-11.15.0.1594, a beta version）,display normally.

How can i solve the problem ?

 

PS: not good at in Englsh, sorry.

[LexManos]

Minecraft rendering logic expects blocks to be shaped a certain way. We fix it to include stairs for performance reasons.

Follow Diesieben's instructions if you INSIST on breaking Stair's shapes.