Custom villagers are a large part of some maps, and if you summon a new one, they do not work properly. Either they replace their trades, or their UI immediately closes after interacting. This is not fixed in the 1.14.4-snapshots, as they only affect naturally generated villagers.

Reproduce:
1. In any new world, type /summon villager ~ ~ ~ {Offers:{Recipes:[{maxUses:10,buy:

,sell:{id:"emerald",Count:1}}]}}

2. Its UI will flicker when you interact, and you cannot trade with it.

Setting the followRange to 0 (which disallows workstation shifts) does not work either. There seems to me no way to summon any custom villagers, and that breaks quite some maps.

This might be a duplicate, but it's not MC-154201 as it is not fixed in the snapshot.