Vanilla examples are buttons and levers. Because they use the "x" rotation argument in their block model files, the e/w wall variants do not have shading applied.
MC-52540
Lack of Shading on East/West Block Variants Using The "x" Rotation Argument
Resolved
Fixed
3
(Unassigned)
Unconfirmed
model, rotation, shade, variant
Minecraft 14w11b, Minecraft 14w17a, Minecraft 14w18b, Minecraft 14w19a, Minecraft 14w20b, Minecraft 14w21b
Minecraft 14w25b
Retrieved
Could you test if this is also a concern without using a resource pack?