All villagers with saved memory disappear when upgrading to 20w07a

Confirmed!  Only the villagers have disappeared, over 40 of them, Large number of errors like this in log:

[08:31:11] [Server thread/WARN]: Exception loading entity:
q: Loading entity NBT
at alf.f(SourceFile:1599) ~[20w07a.jar:?]
at alj.a(SourceFile:441) ~[20w07a.jar:?]
at alj$$Lambda$3198/1274024699.accept(Unknown Source) ~[?:?]
at t.a(SourceFile:341) ~[20w07a.jar:?]
at alj.a(SourceFile:440) ~[20w07a.jar:?]
at alj.b(SourceFile:493) [20w07a.jar:?]
at alj.a(SourceFile:474) [20w07a.jar:?]
at cdm.a(SourceFile:375) [20w07a.jar:?]
at cdm.b(SourceFile:138) [20w07a.jar:?]
at cdm$$Lambda$3081/1202186867.accept(Unknown Source) [20w07a.jar:?]
at cda.w(SourceFile:478) [20w07a.jar:?]
at ws.a(SourceFile:603) [20w07a.jar:?]
at ws$$Lambda$3184/1876020038.apply(Unknown Source) [20w07a.jar:?]
at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) [datafixerupper-2.0.24.jar:?]
at com.mojang.datafixers.util.Either$$Lambda$3185/192084021.apply(Unknown Source) [datafixerupper-2.0.24.jar:?]
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) [datafixerupper-2.0.24.jar:?]
at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) [datafixerupper-2.0.24.jar:?]

Confirmed too. Did a little experiment in a creative world, found a village and upgraded the world.

Result: I found only 2 villagers.

This happened to me too! Luckily I made a backup of my world before updating.

Same thing. Only affects villagers with profession, all biomes.

Affected all villagers in my world, both employed and unemployed.

Mine I had in the Nether disappeared too.

All villagers wiped out in 20w07a. Tried with several worlds from 20w06a, same result.

I got the same thing, I tried going into the nether and saving and updating from there, saving again after updating, then traveling to the overworld away form the village. I then get the error when I travel towards the village in the overworld.
hope this prompts a b snapshot.

The same happened. All villagers have gone...

In my case only dissapear the villagers with profession. The villagers unemployed are safe. 

I might be wrong, but i also noticed that you are required to break and place a job site block or a bed again in order to make a villager (cured from zombification, might it) get a profession or get to sleep respectively (before, this was automatic and they would automatically become). This was from my experience but i might be wrong about this.

In the while, you could go in Creative mode if you have cheats enabled and remake your village accordingly, but this is a bummer nevertheless. Is this the first time this bug happens?

Same here. They all disappeared.

Can confirm, from 20w06a to 20w07a. Cannot confirm, from other version to 20w07a.

Exception:

Villagers spawned by spawn egg don't despawn upon same upgrade. But they despawn upon same upgrade after their inventry or jobs are changed. 

Seems like the error happens in net.minecraft.world.entity.ai.memory.ExpirableValue#<init> (seems new, couldn't find it in 20w06a) in this code block:

public ExpirableValue(Function<Dynamic<?>, T> var1, Dynamic<?> var2) {
      this(var1.apply(var2.get("value").get().orElseThrow(RuntimeException::new)), var2.get("expiry").asLong(9223372036854775807L));
   }

The method is called from net.minecraft.world.entity.ai.Brain#readMemory(net.minecraft.world.entity.ai.memory.MemoryModuleType,com.mojang.datafixers.Dynamic)

private <T, U> void readMemory(MemoryModuleType<U> var1, Dynamic<T> var2) {
      ExpirableValue<U> var3 = new ExpirableValue((Function)var1.getDeserializer().orElseThrow(RuntimeException::new), var2);
      this.setMemoryInternal(var1, Optional.of(var3));
   }

and that is called from net.minecraft.world.entity.ai.Brain#<init>

private final Map<MemoryModuleType<?>, Optional<? extends ExpirableValue<?>>> memories = Maps.newHashMap();
...

   public <T> Brain(Collection<MemoryModuleType<?>> var1, Collection<SensorType<? extends Sensor<? super E>>> var2, Dynamic<T> var3) {
...
      Iterator var4 = var3.get("memories").asMap(Function.identity(), Function.identity()).entrySet().iterator();

      while(var4.hasNext()) {
         Entry<Dynamic<T>, Dynamic<T>> var5 = (Entry)var4.next();
         this.readMemory((MemoryModuleType)Registry.MEMORY_MODULE_TYPE.get(new ResourceLocation(((Dynamic)var5.getKey()).asString(""))), (Dynamic)var5.getValue());
      }

looks like something is feed, where the value-Attribute is missing.

Memories of a villager (from 20w06a) that is lost in 20w07a:

Log file of 20w07a:

Can confirm this also happened to me, I went back and checked the log file, and it's full of "exception loading entity: q: loading entity NBT" as others have pointed out. since this is an entity loading error, do the villagers still exist in the save and can't be loaded, or are they permanently thanos-dusted? I have cheats on in my world for this sort of eventuality, so I'm not in too much of a pickle if I have to train them all up again. However, if they still exist in the save and can be restored in a future snapshot, that would save me some work.

@SaturnoCane92 That's actually a different Issue and has been affecting Villagers since 1.14.2 MC-160250 

Later on Mojang adressed this problem, unfortunately Mojang only fixed the Issue for Cured Villagers/ Villagers summoned with commands or Spawn Eggs, now even in 1.16 only affects naturally generated Villagers, still very annoying since most of the encounters with Villagers is through natural generation.

I am having the same problem.  Or...the Iron golems in my curated and protected village apparently had a Bolshevik revolution, seized the means of production from the villager bourgeoisie, and then did them Marie Antoinette style to appease the masses.  They are now marching through town in formation and having parades while waving copies of Karl Marx's and Mao Tse Tung's various manifestos.  Please tell me the "b" version of the snapshot will Marshall Plan them back into existence.