I have always been frustrated in trying ti perpetuate stamina in my breeding.
I started a stable intended only to perpetuate horses with high stamina. only horses with high stamina were permitted to breed. After several seasons of breeding and line-breeding, I began to come up with some yearlings who had stamina similar to their parents. after they debuted as two-year-olds, the stamina was all but gone. I know there are variables in breeding horses and you don't all breed true.
In no other status bar have I noticed a foal being born with one bar as a yearling, and having another bar as a two-year-old.
Has anyone else observed this?

In a flat save I don't think I have had a foal maintain a high stamina when at a young age. My yearlings who have been bred for distance have about 5% at 1yo, then as they turn 2yo it may go upto 10% and by end of 2yo career maybe as much as 30%. End of 3yo career maybe 65% but at 4yo or 5yo the stamina bar tends to go up again to near max.

My combined game, where I tend to concentrate on NH horses, has similiar trends when the horses are younger, but they take longer to max out stamina. Sometimes as old as 7yo.