Hello!
Your first question may very well be, "Peter, what in the nine hells did you do?" and so for this I will try to explain!
While building up my scene, I happened upon a situation where a mesh that I expected to be present, was absent in the rendered viewport.
After visual inspection with the debugLayer, the object was truly in the scene, but simply not visible.
Upon calling mesh.getAbsolutePosition(), I was shocked to see the return value of { x: NaN, y: NaN, z: NaN }
What is odd about this particular situation is that the mesh is one of 10 that are generated in a loop; the others are fine and appear as expected, but my missing mesh (#3 in the list) is visually not appearing.
I broke down my code to determine where this might be occurring, and it seems that, until the following call, the absolute position of the entity resolves to { x: 0, y: 0, z: 0 } (as expected)... once the call is made, the absolute position of the mesh appears as { x: NaN, y: NaN, z: NaN } !
engine.runRenderLoop(this._runRenderLoop.bind(this));
It would be difficult for me to explain in code how things are set up, so I'm hoping that someone might have encountered such a situation and be able to say, "hey, I know what in the nine hells *I* did, so here are my experiences and how I fixed it!"
Regardless of the outcome, I will try to explain my solution when I come across it. 
Thanks for reading!
- Peter