1. Teleport/walk (for chunk loading) to coord: -236, 71, 277.

2. Go below the igloo.

Result: When I played this first, there was no zombie villager. I copied the seed and teleported there instantly, and the zombie villager was there. I also teleported 255 blocks up to see if it was despawning, and it did not. I did explore near this area previously, so possibly some chunk loading/unloading, but the structure is within the chunk. I did have some lag spikes just before this, so that could also be a cause.