Obstructed FVIR window on water heater burn chamber

What does it mean when the glass window on an FVIR-sealed burn chamber of a tanked water heater is obstructed from the inside with a white residue? It’s a 4-year-old Rheem.
Here’s the exhaust pipe for the water heater on the roof:
Here’s the draft diverter for the water heater:

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It looks like condensation.

Do you think that the reduced diameter of the exhaust flue on the roof could have something to do with that?

I see similar white residue on the gas wall and console furnaces I inspect.
Those the glass comes out (though often with an asbestos gasket LOL). The white power seems to be a gas combustion thing, usually associate with a pilot light that’s way too high… can’t be more use than that.

I agree with Martin.