Villager trades increase price when trading (and doing nothing else)

Fletcher trade before trading: 13 String = 1 Emerald

Fletcher after trading (without hitting or killing any iron golems or other villagers): Same 1 Emerald now costs 24 string

Note: It's not just Fletcher's that have this problem. I've seen it with just about EVERY villager, and it tends to only effect one trade per villager, while things that are supposed to increase costs (like hitting villagers or loosing the discounts from multiple trades or "Hero of the village"), affect most or all trades.