The bug

When specifying an NBT path you're unable to let a compound be followed by a list after specifying the index.

Examples

(1) /execute if data entity @s SelectedItem{id:"minecraft:stone"}
(2) /execute if data entity @s Inventory[0]{id:"minecraft:stone"}

(1) is allowed and tests whether the SelectedItem of the player is stone, (2) is not allowed but should test whether the first compound in the Inventory list has an id of minecraft:stone.