Villagers Limit & Vanishing from the Nether

Firstly I seem to have reached a limit of villagers in my trading hall. Any new villagers I attempt to bring in, no longer link to beds or workstations. I have 200+ in a 100 x 100 area. As a result I decided to expand the trading hall to the Nether: I transport villagers from my breeder through a portal where I drop the villagers, still in a minecart, over a 100 blacks into the Nether trading hall. I place a bed and a workstation that the villager links to fine. The moment i release the villager from the minecart however, they immediately vanish. I have been able to find them again, in the OVERWORLD side of the portal i brought them in through, over 100 blocks away in the Nether from where I released them.