Given the example advancement, placed in world/data/advancements/custom/full.json:

{
    "criteria": {
        "findme": {
            "trigger": "minecraft:inventory_changed",
            "conditions": {
                "slots": {
                    "full": 8
                }
            }
        }
    }
}

And the following command to test it at will (see MC-115064 for namespace issue):

/advancement test @p minecraft:custom/full

Updating the inventory with exactly 8 slots containing items will complete the advancement, even when the slots are not full. The game is checking if the "max stack size" is equal to or higher than the "current stack size", which is always true. Simply reversing the two will fix that.