I was messing around with the warden and spawners until I saw that the warden is invisible for like a frame when it is spawned in. I saw this because I had hitboxes active and I could see the warden's hitbox appear before the model loads in.

To repeat this, Grab a spawner and right click it with a warden spawn egg, then, turn on hitboxes and wait to see that there is a moment where the warden's hitbox is visible but the warden itself isn't.

I would try to screenshot it, but I tried countless times and couldn't time it just right.

Just to note, I tried this with other mobs and it doesn't seem like they have this behavior when spawning in.