When you run hard, then suddenly stop, the motor loses all wind cooling, and the inside temp will spike more than you might think possible. When I fried halls in smaller motors pretty regular, they typically died while the motor was stopped, despite my attempt to run a bit of cool down time at low speed and power. Basically, once outside temps got to about 40c, the motor would not cool enough, and I'd fry the halls in a motor. Why would cooking off your halls not cook off all three? Assuming they did cook off.
Even very small, like 3 mm holes in the hub cover can help a lot with quicker cooling of the motor when you stop, even if pretty much totally ineffective while the motor runs. It also lets you smell it when you stop, when you are cooking off the windings. poor mans temp monitoring, stop when you smell fairly toasty windings, before you smell burnt.