The two governing factors here are likely padding and heat. If they're only wearing one leather article of clothing, it would be under the mail to provide extra padding to both further spread out the force of a blow and to provide better protection in the event a blow got through the chain links.

If they're wearing a padded aketon/gambeson and then mail, and then a leather waistcoat, that's basically wearing two winter coats with 20 lbs/10kg of weight in the middle. The heat load is going to be severe.

In battle, if they're wearing their knight's colors or badge, you'd be more likely to see it on a shield, a strip of cloth tied around an upper arm, a light surcoat/tabard, or a hat/helmet badge. But, they're not going to be in full harness most of the time, and a leather coat with a badge could be reasonable "around town" uniform/light armor, just like they might carry a large-but-not-huge knife instead of their battleaxe when going into town. As.a garment, it's an extravagant use of leather, but it's not crazy.