Ah, the classic "we just found a bug in something you built during the Obama administration" scenario. That forced smile hides the internal screaming of every developer who's had to dive back into ancient code they don't even remember writing.
The best part? The feature probably worked perfectly for 4 years until someone decided to use it in a way that defies all logic and reason. Now you get to archaeologically excavate your own code while the client watches with that "we're paying you a lot of money" expression.
Time to dust off the old commit history and figure out what past-you was thinking... if you even documented it. Spoiler alert: you didn't.