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.
- relates to
MC-72774 Entities render incorrectly on the first frame after being summoned