Why is my object disappearing at runtime?

Hi, too few infos to be sure this is your case but since I had similar issues..

1. Find the Animation component in your object. There you should see the Culling Type option. This is set by default to Based on Renderers which basically means that if your object goes out the rendering context it will no longer be animated. If your object is out of the scene when the game start, you'll not see it. So, if you want your object to be animated bypassing any logics that Unity does to optimize animations, set Culling Type to Always Animated. Have a look to the other conditions too as they could meet your needs.

2. Make sure you don't have any script that changes your object's layer to another one that might be "culled out" and thus not rendered.

My Animator models used to disappear and by right clicking on the rotation property and changing it from Euler Angles to Quaternion fix it.

It might be worth a shot, but have you checked in your modelling application of choice whether the surface normals are reversed? That could be a reason why the object doesn't show up while the others do (given the same script and same scene used). Does it only happen at runtime? Have you checked up close in your scene view?

There are many related questions available to surface normals. Try this Answer

I know you said that all procedures are the same, but did you change anything with layers? Is it truly rendered by the correct camera? Is the object so close/far away to the camera, that it's outside of the the clipping range? That's all I can think of for now.

So I wanted to add to this because I tried the solutions above and still no luck!

Turns out that the Skinned Mesh Renderer I was using had "Update When Offscreen" unchecked! Once this was checked the animation and model stopped vanishing when it was half off screen. I hope that helps someone else in the future.

My issue was the camera deleted some Clear flags and culling masks were set to only some filters.. try setting it to the right mix or everything and skybox..