When villagers acquire enough experience to level up more than once, they only gain a single level

When villagers gain enough experience to level up multiple times (which is possible using certain trades), they'll only ascend one level. This makes it possible for villagers to stay at a certain level despite having a full experience bar, requiring further trading to update the villager to its proper level.

To reproduce, employ a villager to a stonecutter, unlock the mason to apprentice level, gather multiple stacks of stone, and sell the stone until the trade is maximized. Notice that even though the villager is only a journeyman, it has enough experience to be even higher level.