Pretty straightforward:

While models' texture references are formatted assets/<namespace>/textures/<name>.png, advancements' background texture references are formatted assets/<namespace>/<name>.png.

The advancements' background texture reference path should be changed to include the "textures" folder automatically for consistency. The current behavior is incorrect, as the models' is consistent with sound, model, loot table, function, tag, advancement, dimension, dimension type, structure and feature references.

Examples:

Model:

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "minecraft:item/acacia_boat"
  }
}

Texture path: assets/minecraft/textures/item/acacia_boat.png

Advancement background:

{
  "criteria": {
    "entered_end": {
      "conditions": {
        "to": "minecraft:the_end"
      },
      "trigger": "minecraft:changed_dimension"
    }
  },
  "display": {
    "announce_to_chat": false,
    "background": "minecraft:textures/gui/advancements/backgrounds/end.png",
    "description": {
      "translate": "advancements.end.root.description"
    },
    "frame": "task",
    "hidden": false,
    "icon": {
      "item": "minecraft:end_stone"
    },
    "show_toast": false,
    "title": {
      "translate": "advancements.end.root.title"
    }
  },
  "requirements": [
    [
      "entered_end"
    ]
  ]
}

Texture path: assets/minecraft/textures/gui/advancements/backgrounds/end.png
Expected with this input: assets/minecraft/textures/textures/gui/advancements/backgrounds/end.png