^{Edit by zTxrbq:}
It seems that friendly creatures like new wolf variants and armadillos don't spawn at all in old (upgraded) chunks, not even in new chunks, only once in newly generated chunks.

Steps to Reproduce by kelianor in this comment

1. Create a single-biome savanna world in 1.20.4 with commands enabled
2. Set simulation and render distance to 16 or more.
3. Once you load in a world, fly around spawn to load new chunks.
4. Upgrade it to 1.20.5
5. Place 2 always active repeating command blocks with commands bellow

   /execute as @e[type=armadillo] run say MC-271088 is false
   /execute as @e[type=armadillo] run effect give @s glowing infinite 
   

6. Run /tick sprint 2d and stay still.
   Armadillos don't spawn.
7. Run /tick sprint stop otherwise chunks might not load
8. Run /tp @s 30000 320 30000 
9. Repeat the same steps in 5, 6 points.
   At least one armadillo shoud have spawned
10. Wait until all chunks are loaded and run /kill @e[type=armadillo]
    New armadillos don't spawn.

Note by SeacraftGaming in this comment

I originally thought that this was intended behavior, and occurs because of something called a "mob cap", which prevents mobs from passively spawning after reaching a maximum capacity. For friendly creatures (which includes armadillos), this cap is only 10, and is likely already filled by existing mobs in the area like sheep, pigs, horses, etc. Therefore, no additional friendly creatures can spawn. On top of that, friendly creature spawns are also quite infrequent.

I ran steps 1 and 2 as instructed (in 1.20.6), and, sure enough, nothing spawned. However, I then ran /kill @e[type=!player] to kill all the mob-cap-filling mobs. I was expecting some creatures to spawn, maybe run the kill command a couple more times and wait until an armadillo spawns, putting this report to rest. However, I found that, despite waiting multiple in-game days, no friendly creatures spawned at all. It is possible that friendly creature spawns are just that rare, but I am highly doubtful of that. It is also possible that this is a savanna-specific issue, but I am not sure and do not have the time to test it right now, and I do not see any reason for that to be the case.

Old Description

With the Armadillo // Wolf Armor Update officially releasing yesterday, I assumed that I would be able see these little critters and new wolf variants on my Minecraft realm once it updated.

Unfortunately, that didn't happen. Despite traveling to the Savannah biome on my server, no Armadillos spawned. I tried downloading my realm to my Minecraft single player mode as an attempt to sync the update into my realm. Once that didn't work, I tried optimizing my world. Once that didn't work, I attempted to load back into Minecraft via the Installations menu and customizing a "most recent launch" through there. I tried loading back into my Realm World once more and.... nothing. No armadillos; no new wolf variants.

If you want to try this bug out for yourself, just try to find an armadillo on your realm in a pre-existing Savannah biome. They won't be there. And if they are, please let me know what I am doing wrong, because I have tried everything aside from deleting this world from my realm (which I would prefer not to do since I've had it for 2+ years now.

Thanks.