Interesting. The only machine I've ever used is my Office Control. During 30 seconds or so of steaming, I always get at least one boiler-fill moment, and often get two. And I always use the hot water tap to force a fill beforehand to open my bleeder valve. Apparently I have a very small hysteresis band compared to the Evo's (I'm not complaining. And I have noticed that the hysteresis band gets larger after de-scaling the boiler). I have never detected a change in steam pressure during steaming due to a fill event. With so little water added each time, the effect on temperature must be negligible. (The downside of this is that I can't steam and pull shots at the same time until I add a relay to prevent those fills during a pull). I do notice that the steam pressure is a little lower at the end of 30 sec. than at the start, and the heating element stays on another 30 seconds afterwards to finish the recovery. This seems consistent with the amount of milk heated and the (small) size of the heating element.
-Ed