It's not necessarily a war room bug. Mind you, this bug didnt exist until either changeset 18 or 19, so something in that changeset created this bug. It's only about a year and a half or two years old.

In any circumstance of reverse boom, (i.e. "your people demand a tax rebate" or "a drought occurs lowering your food production"), when the game does the math for those it takes your food or income production × 1/3. What displays after will often be .3333333 or .6666667 depending on the actual math. Clicking anything, like literally any button, will cause the math to correct and round in the proper direction. I've seen it occur on both the build and war screens.

It isnt just limited to the war room, but in the war room, standard turns can cause the non-rounded amount to show. Most typically tho, it is a bug that is a product of droughts and tax rebates, not necessarily a war room bug. I would bet the farm on the fact that he had a reverse food boom and was not necessarily in his war screen.

The oddest thing about it, is that in fact your cash income and food income are based on, at times, long decimal points. And the rounding bug occurs after those numbers are rounded. I'm going to be bad at explaining this, but I will try.

Let's say I produce 12,343.4 food per turn after my tech percentage is figured in and consume 500. If I have a food fail, the game will round my production down to 12,343 and then times it by .33333. And then finally it will minus consumption. So you will end up with 3,614.33333333 food being added to your total, and then clicking anything will remove the rounding error food of .333333. So you can lose like .8 of a food to rounding errors in one turn lol.

Negative boom feels like:
Round(Round(food or cash production) × 0.3) - Round(consumption)))

When in reality the formula should behave like:
Round(food or cash production × 0.3 - consumption)

It's like waaaay overcomplicated for no reason