The lighting for stairs gets messed up when the stairs are in complete darkness and placed over a light source

Appears in both 1.15.2 and 1.14.4

1.15.2 Modlist:
Forge 31.1.0
SnowRealMagic 1.7.4
Kiwi 2.6.5
Mixinbootstrap 1.0.2

1.14.4 Modlist:
Forge 28.2.0
SnowRealMagiv 1.5.2
Kiwi 2.6.3
Mixinbootstrap 1.0.2

I think it is a vanilla bug. If you look carefully to vanilla stairs, you can see same issue there. My blocks is a little special and make this more obvious

I do not know how to fix this, sorry