Villagers cannot be transported through portals without two people, one on either side of the portal. The problem is that I have not found so far any other way to move them through except by walking into them and pushing them into the portal, this means that they have no momentum going through.

So when the player then goes to the nether through the same portal, upon loading the nether, the game recognises the villager being in contact with the portal, and transports them straight back again, leaving no chance to make them remain in the nether. This does not happen in the overworld, as there is a small delay in which you can move them out of the portal before they teleport again.

However, if one person is in the overworld, and one is in the nether, this causes the delay in transport between the realms to occur in the nether as well.