Villagers decrease prices when players saparate trade into multiple times

How to reproduce:

1. Set time to 18000(midnight) to stop resupply
2. Summon two villagers (Villager A and Villager B) and turn them into cartographers
3. Buy 10 empty maps from Villager A
4. Buy 5 empty maps from Villager B (and close the trading interface)
5. Buy 5 empty maps from Villager B again **(and close the trading interface).
6. Villager A continues to sell empty maps for 7 emeralds, while Villager B decreases the price to 6 emeralds.

I think this behavior is inconsistent.

Similar problem happens to other kind of villager.  Unlocking of trades seems not relevant.