XP is client sided. Treyarch doesn't care about hackers/cheaters unless it's losing them money like free Black Market tiers/CoD Points etc...

I know it's off topic, yours is just a visual bug but I was able to add 1439600 (Max XP + Prestige Available) after a game of MP and it worked just fine lmao. TAC (Treyarch anti-cheat) is inefficient. They take screenshots of your screens on launch, used to detect things like overlays for ESP/Wallhacks, but by simply using the Steamoverlay (Adding a Non-Steam game to Steam), you're overlays are whitelisted and jumped by TAC. OR, you can simply suspend the thread at entry 0x1CCEA0, which also happens to be the Anti-Debug thread as well, so once suspended it allows you to attach a debugger, I.E. Cheat Engine, ArtMoney etc...

This game dookie.