This is reported occasionally. It usually happens on the ultra long routes. There have been examples posted this year from LAX, and I think SFO.

15kg is your ticketed allowance in business, so it's hard to argue. When they let you take more they are doing you a favour, technically.

The fact that it is reported so infrequently shows that it's not a standard policy, but enforced when the flight requires it. There could be one or more of a full load, stronger headwinds, other weather issues requiring a change of flight altitude, some sort of war or weather diversion required, etc. All of which can affect fuel consumption, which could be an issue towards the limits of the aircraft's range.

Now you could argue why it matters whether the weight is in the hold or the overhead lockers. And the airline might suggest it allows them to allocate weight more evenly, or to charge those who are maxed out on their checked-in allowance and also over the hand luggage limit to cover their extra fuel costs. And it's probably simpler and quicker to enforce the published allowances for everyone than to have hundreds of sepatate debates about combined checked plus hand luggage totals. We're all special cases, but not really

Either way, it's unusual and everyone is still getting what they paid for.