Ever seen the aftermath of a house fire? Sometimes things you’d never expect make it through ok, and other stuff is completely destroyed. Fire doesn’t operate by a simple set of rules…
Fire does have a set of rules ... and behaviors. It's how fire inspectors determine causes and origin points.
Rule #1 -- Fire needs 3 things to exist: Fuel, heat and air/oxygen
Rule #2 -- Fire will follow the path of least resistance where heat, fuel and air are available.
if you browse through any scientific study of fire, you'll see age-old discussions on the chemistry, properties, behaviors and variables that affect fire.
https://www.nist.gov/el/fire-research-division-73300/firegov-fire-service/fire-dynamicsGiven that fire is predictable if the variables are known, it's more than possible, for instance, for a home to be utterly incinerated while leaving small, often unprotected, items or areas seemingly untouched.
As fires burn, they consume the fuel available. I.E. the structure changes. As doors are destroyed, windows broken and roofs collapse, the balance of fuel and air is always changing.
In the case of a passport, what if it's in a wooden box inside a drawer inside a desk? By the time the fire reaches the inside of that wooden box, the fuel, air and heat may have been reduced to such a degree due to rapidly consuming the rest of the house that it can't sustain itself to the same destructive level.
It's difficult to explain why one thing burns totally while another is not burned the exact same way. Unless you reenact the fire with the same environment and circumstances, it's a futile debate.