The light level displayed in the item's texture does not match the one in the world.
If you do the command /give @s light{BlockStateTag:{level: 8}}
It gives you a 15 light-block item. Although this is wrong and the command is supposed to be ran with quotes around the level number, when the item is placed in the world, the level is different than the displayed in the item (See Image 2)
This means that the level is getting computed, but due to some parsing error, the wrong texture is being attributed to the given item.

Old description:
The light item displays the wrong level number when the command to get it is incorrectly done, although the block when placed works just fine. 

Steps to reproduce:

• Do the command: /give @s light{BlockStateTag:{level: "8"}}
• Notice the texture is correct.
• Do the command: /give @s light{BlockStateTag:{level: 8}} (without quotes)
• Notice the texture is incorrect.

Image 1: Product of the commands above in my hotbar.

Image 2: Proof that the level is correct in the world besides the item displaying 15 (default)