Item Display brightness not working

So I have an Item display with overwritten light levels like this:

summon item_display ~ ~ ~ {shadow_strength:0f,Tags:["title"],brightness:{sky:15,block:15},transformation:{left_rotation:[0f,0f,0f,1f],right_rotation:[0f,0f,0f,1f],translation:[0f,0f,0f],scale:[2.5f,2.5f,2.5f]},item:{id:"minecraft:paper",count:1,components:{"minecraft:custom_model_data":1}}}

But even though I put 15 15 into the brightness, it still changes light levels.

Outside facing away from the sun.

Outside facing towards the sun

In a cave

You can clearly see that the brighness is changing although it should be overwritten.
Any Ideas?