Boilers of this sort of ilk tend not to be (auto) modulating. Of hand the only natural draught boiler with modulation I can think of is a Vaillant Sine (1980s combi).
No I mean on most ordinary burners. You can see the effect on ovens, grills and sometime hob burners - it's a transient effect. I presume that the gas rate increases immediately but there is a transient delay in the corresponding air flow increase into and through the mixing tube(s).
This is a somewhat simplistic analysis. The point I'm getting at is that the flame temperature is constant (due to the air/gas ratio being more or less constant). The excess air forms a bigger proportion of the total flow through the boiler at lower gas rates. There will be some variation in the flue gas temps with gas rate, this effect will be to mitigate the effects of the excess air.
The point I was trying originally to make was that modulating burners with fixed speed flue-fans are not really all that bad. They are of course not as good as premix burners with tightly controlled variable speed blowers. The gas valve on those boilers, sometimes known as a zero-governor, is something akin to a carburettor, namely to keep the fuel:air ratio as constant as possible over a wide range air flow rates.
I don't know, but I strongly suspect that the 'carburettor' on an LPG vehicle would be much the same technology.