4 2x2 blocks wide and 3 blocks high containers were made to contain villagers. Water was added to the floor and a torch to an upper wall. 3 villagers were added to each container and then it was sealed off (no light apart from the torch. Each container was identical and each was given the same number of villagers.

Doors were added near the villagers to count as a village for the purpose of spawning iron golems.

Over several days of game play (real time) many stacks of iron were collected. Yay. Then the spawning rate of the golems appeared to change.

Nothing had changed in terms of doors, other villages, etc.

When I looked in the containers, 2 of the 4 had only 1 villager and the other 2 had probably a dozen or so.

I built a second golem farm and filled 2 of the 4 containers with villagers and only added about half the needed doors before leaving it for a while (to grow more wood for doors). When I came back and checked, both containers had only 1 villager.

This issue appears to be reliably duplicated.