Multiple jmods over the years have asked players reporting this bug to try to explain the exact circumstances under which this happens. I guess they either were never able to recreate it in test conditions or found it was too complex to fix. It's happened to me only by the time I'd need to renew my yak a second time (IE 3 yaks). So usually I just bank my bob, dismiss the current (2nd) yak, even at like 20 mins remaining, and summon a new one.
12-Jan-2016 19:15:20