In version 1.16 0.3.7. and 1.16 0.3.8. stairs have transparent holes that should be opaque after applying a texture to them. This bug was not present in version 1.16 0.3.6.

Technical Information

Minecraft 1.16
Forge 36.2.2
Optifine 1.16.5_HD_U_G8
BlockCarpentry 1.16-0.3.8

Fixed with 0.3.9...

Yup, I already noticed it and I'm currently trying to fix that. Also, some parts of the textures have moved and some faces look weird. I wonder why, since I haven't touched the model files in a while and I only made transparency changes in the last update... Well, I'm trying my best to fix this soon...
Thanks for reporting!
Regards, Manu