The Bug: 

After the player acquires Hero of the Village Status Effect the player will receive gifts from Villagers. This happens after 30 seconds if the player is within 5 blocks. Normally, gifts have a cooldown of between 30 seconds and 5 minutes 30 seconds. However, if you unload the villagers after they throw a gift, this cooldown is reset guaranteeing that the villager will throw another gift at 30 seconds when the villager is reloaded. This can be used to exploit the gift mechanic to get huge amounts of gifts.

How to reproduce:
1. Give the player Hero of the Village effect

2. Find a village or spawn a villager (or villagers) and wait for it to give you a gift.

3. Build a nether portal next to the villager(s) and go through it, wait 30 seconds for the chunk the villager is in to unload then go back through the portal to the villager(s)

4. Observe that all villagers who gave gifts forgot that they just threw player a gift and will throw another gift in 30 seconds rather than a random time between 30 seconds and 5 minutes 30 seconds.