Started a new combined game and my first two 100% horses just turned 7yo with 98% full-filled potential. One was a 2 mile stallion and the other a 2m 4f stallion.

The 2 mile horse won 6 G1 wins from 6 outings including the Champion Hurdle and the Queen Mother Chase. The 2m 4f horse won 5 G1 races from 6 outings. They were winning their races by about 5 lengths. I was over the moon with their performances and decided they were going straight into the breeding barn. This was going to be the start of something special.

Bad luck struck, at the turn of the year my game went wonky, suddenly there were 20 copies of the same yearling in the breeding barn and I couldn't get rid of the copies.

I had a copy of the game saved away to a USB drive, but it was 2 seasons old. No problem I thought, the two good horses were in the stable as 5yo. I played the 2 seasons until the horses turned 7yo. I don't know what I did different catching the game up 2 seasons but this time both horses were 100% full-filled potential. Even better I thought.

Sadly NO. They only managed to win one G1 races between them and their performances in the other races was poor. Often finishing 7 lengths behind the winner.

I don't expect there to me an exact mathematiclal formula that is exactly the same everytime. Horse racing is full of unknown variables but I wasn't expecting such a large difference over the 2 seasons with the 2 horses.

What is even stranger is they had a couple of half brothers who only had a 95% potential bar that was fully filled at 95%. The half brothers who were usually in 2nd place in the first game were now winning a few G1 races in the 2nd game, beating their more illustrious brothers.

Just before the problems with the first game, I had exported the two horses as 7yo to the race kit. I couldn't export the 2nd versions of the horses as it would overwrite the originals but I exported the 2nd two horses as 8yo and they were significantly poorer than the original 7yo horses.

At the start of the game I found this happening. I just retired them and tried to breed horses who kept their form for longer. In your case the second game horses may have kept form better, pace of race may have been different, jockey's different or ground different. A load of variables. From running extensive trials I know the addition or subtraction of 1 horse from a race can drastically alter a result. A really good horse is one that can win under all conditions

Also I find if a horse loses form it's performance suffers in some cases. Best to stick it in a handicap and get it winning again. There is a trait you can't see anymore in league mode ( Confidence ) and your horses may be low in this. If this is the case probably best to try breed it out as it will suffer in the league

The yearling thing is just a display bug. Ignore it only 1 copy will turn 2. Happens to me all the time. Even posted an image or too in the bug thread.

Try appoint a stable jockey as well ( if you haven't already), as some jockey's ride your horses really badly and affect performance