Over time, villagers, for some reason, stop breeding and farming. Farmers refuse to pickup seeds when thrown at them and they do not tend to crops. Villagers do not breed when they haven't met the cap, presumably because they have corrupted inventories - farmers should throw food at them unless they're willing already, but that isn't happening or they'd be picking up crops too. Trading seems to be the only way to repopulate, and with baby zombies staying alive in daylight, my villagers are being slaughtered en masse with no way to perpetuate quickly enough. As soon as they're traded with a few times, they breed again provided the cap is high enough.

Willingness just doesn't work. Giving up on villages and spawning custom, trapped villagers until it works.