Villagers when trying to cell them , they would stand on their beds as no other blocks are valid to spawn after sleep (except 1 side which has a gap to trade and below it is their workstation ),  now they are standing on the workstation and suffocating to death as there's a solid block above the gap to prevent from escaping

Side note : out of like 12 villagers only 5 suffocated and other 4 stood in their workstations

The way i used to cell is : their bed surrounded by walls of 4 block height except for one side which is front side , there will a workstation , gap and 2 solid blocks and of course a roof