Block Diplays of certain blocks, rotated with the Rotation tag, have different shading levels. So far I've only noticed this with the following block displays: Mangrove Propagule, the upper half of the Sunflower (specifically the yellow flower), waterlogged Sea Pickle (specifically the stem type thing).

I have made the attached images (with mangrove propagule block displays) showing horizontal Rotations of 0° to 180° and showing 180° to 360°. The only thing changed from each block display to the next is the location and Rotation tag, with Rotation[0] incremented by 10 degrees each display going from left to right. This image was done on 1.20.6, but I discovered it while using 1.20.4. I suspect it might affect earlier versions as well, but have only tested on those 2 versions.

Expected results:

The block displays would have the same shading regardless of their horizontal Rotation value.

Observed Results:

These block displays display varying shading with different horizontal rotation values, based on what seems like a sinusoidal pattern, shifting between light and dark and back.

With the Propagule, the whole plant seems to get darkest around 90° and 270°, brightest around 0°, 180°, and 360°.

With the upper half of the Sunflower, the slanted part with the yellow flower itself seems to get darkest around 0°, 180° and 360°, with the brighest points around 90° and 270°.

With the waterlogged Sea Pickle, the upper white part seems to get darkest around 90° and 270°, brightest around 0°, 180°, and 360°.

Steps to Reproduce

1. Create new world in 1.20.4 or above, with commands enabled
2. Execute both commands from one of the command sets below
3. Visually compare the percieved brightness / shading of each block display. (For the Sunflower, note the yellow flower part. For the Sea Pickle, note the white part on top.)

Mangrove Propagule

/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:mangrove_propagule"},Rotation:[0f,0f]} 

/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:mangrove_propagule"},Rotation:[90f,0f]}

Upper Half Sunflower

/summon minecraft:block_display ~ ~ ~ {block_state:{block_state:{Name:"minecraft:sunflower",Properties:{half:"upper"}},Rotation:[0f,0f]} 

/summon minecraft:block_display ~ ~ ~ {block_state:{block_state:{Name:"minecraft:sunflower",Properties:{half:"upper"}},Rotation:[90f,0f]}

Waterlogged Sea Pickle

/summon minecraft:block_display ~ ~ ~ {block_state:{block_state:{Name:"minecraft:sea_pickle",Properties:{waterlogged:"true"}},Rotation:[0f,0f]}

/summon minecraft:block_display ~ ~ ~ {block_state:{block_state:{Name:"minecraft:sea_pickle",Properties:{waterlogged:"true"}},Rotation:[90f,0f]}