How many mending books have you bought already? I've run into the issue where trading a lot of the same item causes an increase in demand, which raises the price of the item, but the trading window doesn't reflect the change.
This happens to me too. From what I can tell, it happens when the villager walks too far away from their worksite and somehow they desync from it, becoming unable to find and work from it again.
Wilson Balmores
How many mending books have you bought already? I've run into the issue where trading a lot of the same item causes an increase in demand, which raises the price of the item, but the trading window doesn't reflect the change.