Just like in this picture from an old bug. I've tried  isOpaqueCube, isFullyOpaque, every Rendertype and renderlayer.

Maybe try overriding the getAmbientOcclusionLightValue method? You can change the float value from 0F to 1.0F, with 0 being a more concentrated shadow and 1.0 being a more spread out shadow. If that doesn't work, maybe try setting the VoxelShape to empty

@Override
public float getAmbientOcclusionLightValue(BlockState p_220080_1_, IBlockReader p_220080_2_, BlockPos p_220080_3_) {
    return 1.0F; // Fixes shadow
}

Edited March 11, 20205 yr by Techno

Thank you, solved.