To reproduce, click on a sleeping villager's nose (because it has a tiny hitbox) and you could trade with the villager as it sleeps.
Is this a bug, or is trading to villagers so easy that they could do it in their sleep?
MC-146067
You can trade with sleeping villagers
Resolved
Fixed
6
(Unassigned)
Confirmed
Trading
Minecraft 19w11a, Minecraft 19w11b, Minecraft 19w12b, Minecraft 19w13a, Minecraft 19w13b
Minecraft 19w14a
Retrieved
This still happens in 19w13a, but the villagers now get woken up so this may be intended due to this unannounced change.