See title.
Rather than show the light bulb with 0, or 15, it shows the particle that is displayed when held, as the item's texture. When placed, it still places a light that outputs level 15.

/give @s light{BlockStateTag:{level:"-1"}}

The resource pack in the screenshot does not modify lights in any way.

Cause: in assets/minecraft/models/item/light.json, the selected texture is light.png, which is the particle texture, rather than any of the light bulbs; it is normally overwritten via the predicates, but negative values aren't covered. The default texture should instead be set to light_15, as it places a level 15 light block.
See attached file for the current model file.