The bug

Most zombie-related mobs, even including zombified piglins, can zombify/infect villagers, while zoglins cannot. Zombies, husks, drowneds and zombie villagers are designed to be able to zombify villagers. However, zombified piglins(formerly known as zombie pigmen) also can infect villagers(using commands to make them attack villagers). As zombified piglins can infect villagers, it's expected that zoglins, another variant of zombified nether mobs, are able to zombify villagers as well, while they cannot in fact, causing it to be inconsistent.

How to reproduce

Simply let a zombie-related mob kill a villager in hard difficulty(or normal with half chance) to transform the villager into a zombie villager. It's observed that zoglins even in hard mode still kill villagers instead of zombifying them. Zombified piglins can be done using command

/execute as @e[type=zombified_piglin] run data merge entity @s {AngryAt:[I;0,0,0,0]}
/execute as @e[type=zombified_piglin] run data modify entity @s AngryAt set from entity @e[type=villager,limit=1,sort=nearest] UUID

respectively.