I have some mares that are 17 and are born in Y140 (this is correct), but I also have some mares born in Y141 also listed as being 17 when they should be 16. Most of them are not wilds, is this a bug?

It's harder to say because you can't see breeding history since the dam's of the horses you linked have died, but I'd bet it was because they were bred on the first of the month, but before the RO clock hit.