The predicate entity_properties.equipment ignores "items" if one drop the item on the slot

Create a datapack with a predicate like test:1

{
  "condition": "minecraft:entity_properties",
  "entity": "this",
  "predicate": {
    "equipment": {
      "mainhand":{
        "nbt": "{id:\"cpp:broom\"}"
      }
    }
  }
}

Then run

/give @s carrot_on_a_stick{id:"cpp:broom"}

and put it on mainhand, this predicate will pass.

Then drop the item, this predicate will still pass, but now there is no item on mainhand.
 
If one add

"items": ["minecraft:carrot_on_a_stick"],

in this predicate, the predicate works well.

Moreover, if you have two predicates, one for mainhand, one for offhand. Then you drop the item from your mainhand, you can still press F to switch the predicate you can pass.