deception check to contest player's insight check?

hi! so i have a player who LOVES insight checks. any somewhat npc says anything they havent already seen, its an insight check. which is fine obviously, they roll well or they dont and i can vibe with either

​

but say i have an npc, and just a random example, saaay this npc is the secret big bad whos hiding behind multiple red herrings, and is very intentionally deceiving everyone around him. if my player wants to roll an insight check on a thing he says, is a contested deception fair? i think it definitely is but the PROBLEM is i wouldnt really want them to Know its a deception check, because obviously that would tell them that hes lying anyways. my players are good at not metagaming so thats not really the problem but im looking forward to a reveal. so is it bad to hide Why im rolling from the player? id be fair, if they beat my deception check i would accept defeat, but i dont want to seem unfair. if they roll a 19 and i roll a 21 for a random example, i dont want them to later think i just disregarded the 19 from earlier.

​

am i maybe overthinking this and you guys think it would be fine, or would it be uncool to hide the purpose of my roll? i roll without telling them why all the time lol