I don’t know this machine. Is it carburetted or fuel injection?
My guesses therefore won’t be very helpful (Not as unhelpful as, say, an Obnoxious Fat Hippy who knows fuck all about cars and motorcycles and couldn’t care less, but still fairly unhelpful)
Re the Solution (?) above, this is good basic maintenance BUT
Oil and filter are unlikely to make any difference to a flat spot (Most small motorcycles and scooters don’t have an oil filter anyway).
Air filter is unlikely to make any difference to a flat spot early in acceleration. Carburetted engines are fairly insensitive to air filter clogging, and if it has any effect, it’ll be at full power when the air demand is greatest. It’ll be down on full power and will run rich.
Fuel injected engines, (at least on cars, motorcycles may be a bit less sophisticated) are almost completely insensitive to air filter clogging, since the system adjusts the mixture to compensate for the reduced air supply. Full power will be reduced but it won’t run excessively rich like a carbed bike might.
Checking the plug for signs of running rich or lean MIGHT be informative, but a flat spot is a transient state, while the plug appearance reflects the average of the conditions its exposed to. IF you can hold the throttle in the flat spot for a while, then turn off, and pull the plug, its state might be a better clue.
Changing the plug is also cheap basic maintenance and MIGHT make a difference.
So far, so negative. The fact is, there are lots of potential causes, and the fact that its transient will make it harder to diagnose. Here’s an irritating list, produced by someone irritating, on the Internyet. (Not as irritating as, say, an Obnoxious Fat Hippy who knows fuck all about cars and motorcycles and couldn’t care less, yet is perhaps the most frequent poster in Cars and Motorcycles, but still fairly irritating)
One class of PITA (Not as big a PITA…etc) problems that can cause this, and which I have recent experience with on the Skywing, is the vacuum leak. It got better after giving the carb and intake manifold multiple coats of sunflower oil to seal it off.
With older carburetted systems stickiness of the centrifugal advance and/or vacuum advance could cause the ignition timing to be slightly wrong at some points in the rev range. Valve timing and/or worn cam lobes are another possibility.
I once had a car that would hesitate and sometimes cut out on decelleration. Turned out to be because the core of the high tension lead to the distributor was breaking down. (When it started doing it on acceleration too I investigated and replaced it.) Long shot on a scooter but I suppose it could happen.
Again on carbed bikes (I’ve never worked on FI systems and I’d suspect they aren’t really DIY territory) dirt or gunge in particular jets, or a wear ridge on a fuel metering needle, or a failure of an acceleration pump diaphragm, can cause fuel starvation in part of the rev range.
I have some of my own carb cleaning tricks but I guess you don’t DIY, and its probably FI anyway, so I won’t bother with them. Convection ovens don’t feature (aren’t all ovens convection ovens?).
You might try a fuel injector/fuel system cleaner. No experience with them, but Americans seem to like Techron, which I’ve seen at Costco, though maybe local brands (which you can buy at CPC petrol stations in little long-necked red and yellow bottles-dunno what the difference between these is.) are OK too.
The word is it should have P.E.A (polyetheramine) in it, which Techron apparently does.
It could also be that "“They all do that, Sir” (Only in Chinese). If you get a chance to ride another of the same make, you should take it. The flat spot MIGHT just be a characteristic of that system.
OP, I suggest you take your query somewhere else. Cars and Motorcycles ain’t what it used to be.