Minecraft villager rerolls trades even when stuff has already been bought.

Whenever I'm playing Minecraft on switch and PC, sometimes my villagers will reroll their trades even when I've already bought stuff and the work site is not broken? This really sucks because I got mending twice and both times it disappeared before I trades for it. The picture shows a villager that had mending but rerolled it's trade even though I didn't even leave the game.