Custom models - Dark sides when block is facing a certain direction

Hello, I'm having some trouble with custom models. The image on the left is what I'm trying to achieve, and the image on the right is my problem:

When I place the block with my custom model, the right face turns dark depending on which direction the block is placed in. I'm using "Y axis rotation mode." When placed facing north, it appears normal as intended. However when placed facing south, east, or west, I get this "dark side" problem.

I've tried:

• Changing the transparency type.
• Enabling "connected sides" and "transparent parts."
• Changing the size of the bounding box
• Setting light opacity to 0.

If anyone knows of a solution, I'd appreciate it. Thanks.