I'm very happy that someone agrees with me, that it's not normal.
Inconsistency in grind, tamp, distribution etc seem far more likely to be causing this kind of variation than any mechanical factor within your Oscar.
It would be for me either, but this deviation is really consistent. I mean doing a long flush (>7-8 seconds) _always_ leads to this "late start, low volume" thing, and not doing a flush or 1-2 seconds flush always leads to "early start, high volume". And that inconsistency kills me, because 1 second change of flush duration gives me different volume and I feel kind of stupid, when I interrupt the flush when the water is still hissing and boiling.
My newbie guess is that something is wrong with the pump: when it runs without resistance (a coffee puck) it gives low pressure, or something like that. But since I'm really a newbie, and not completely understand how my machine works, I'm trying not to tell anyone my hypotheses.
The complication is that I don't have any local NS services in my country.
And my plan was to make a handmade portafilter pressure gauge and do some measures to prove if there's really something wrong with pressure. Do you think that's a good idea?