I know that can happen when you put a sudden load or are near the inverters limit..
For eg issuing a print command to your laser printer.. which can draw as high as 700-1000 watts immediately.
If the flickering only happens at the time of switcher over and appliances are not shutting off.. i dont think you should give it much thought.
That happens even with bigger inverters.. my 2.5KVA does a flicker at the time of switcher sometimes aswell.