I have that same bug. I had hoped I was just doing something awesome. Oh well.
1. 1473 BC remaining +81 BC actually gain 1701 BC
2. 3097 BC remaining actually gain 1704 BC
3. 3796 BC remaining actually gain 1721 BC

Thanks for reporting this issue !
Did this bug occur after you researched a particular technology ?

If you guys could reproduce this issue and send me the save file to mikedomingues@arkavistudios.com it would be really awesome !

Sorry, not a computer guy. Searched but didn't find the save game. But this is what I did on the game that had the bug.

Play Human, All others AI Very Hard, Huge Cluster Galaxy, Very Far Star Distance, Numerous Average Planets, Rare Debris, Sky Blue Background, Few Pirate Clans, Numerous Minor Races, Conquest & Allied Victory Conditions enabled, Story Events Enabled

Unclick living creature testing. Click propaganda. Click supervised trade. End result federacy. Max out all spending on budget including propaganda. Begin research with scanning. From then go with the least time consuming research with the following conditions. Science is always done first if research times are the same. Military is always done before social if research times are the same.

First things built on all planets are factories based on resource setting: abundant 5, very rich 4, rich 3, moderate 2, poor 1. Second things built on planets are research facilities based on research bonus: 1 facility for every 25% bonus to research. Planets then receive 1 police station if crime is average or lower. Planets receive 1 health care facility if health is average or lower. If only room for one or the other, it goes to whichever is lower. If rating is the same, then it depends on my mood. All other remaining slots become defence towers.

Scout ship is sent out skirting the edges of the solar systems to cover as much ground as possible. As soon as an desired inhabitable minor race planet is discovered a redesigned army ship is built: Marine I. Same as original but with 1 laser weapon. All inhabitable planets of minor races with a resource setting of very rich or better are taken. Redesigned colony ship (add laser: Colonial I) goes out to any inhabitable planet of very rich or better resource setting. Trade routes are opened up with all other minor races. Scout I (add scanner) is built and sent out when original scout ship cannot adequately cover the reachable planets any longer. All starting ship designs are marked as obsolete.

Move shipyard to large population planet with very rich resources. Get rid of old ship yard. Don't spend money on spies. Only take ministers and generals who have extremely high stats, loyalty and favor a federacy.

It was around turn 75 that I even noticed the extra BC because the amounts where so excessive by that time.