Name tags rendered in blocks have letters brighter depending on your angle

The bug

When a name tag is rendered in a block, a letter in that name tag can become brighter depending on the angle you are looking at.

Steps to reproduce

1. Place a pillar 3 blocks tall
2. Place an item frame on the side of the top block
3. Place a block above the item frame
4. Place an item renamed "lightly weathered cut copper stairs stairs stairs" in the item frame
5. Look at the item frame from slightly different angles
   → Notice that one of the letter will be brighter than the rest at different angles

If you are having trouble reproducing, open item frame.zip and active the lever. Look closely at the "o" in "copper" when you get teleported around.

Video

Attached. Notice the letter "o" in "copper" turns brighter.