Fuji lens aperture chatter on X-T3

Hi, there have been a lot of (older) posts about the aperture chatter on some Fuji lenses and cameras, but so far i have not found the exact reason for this scenario:

- Fuji X-T3 on Single AF, (no pre-af), in Aperture Prio mode with a 18-55 lens on f/2.8 of 18mm, both lens and camera on latest firmware

- just point the camera from a dark to light subject (e.g. face forward and lift up to the sky) without pressing any button and look at the aperture blades from the front of the lens (most easy to see at 18mm)

- you will notice that the blades close more and more as the scene pointed at gets brighter and brighter.

- when you then press the shutter button half way the aperture opens to f/2.8 (as expected), and when you release the button, it goes back to some smaller aperture in the audible stepping fashion.

My question is not about the stepping sound and how to avoid it but rather at the WHY:

Why does the camera close down the aperture when you point it a a brighter area without touching any button. At first i thought is was to limit the light reaching the sensor to get a 'well exposed EVF/LCD' view, but that is not necessary: when you half press the shutter (and go to f/2.8) the EVF/LCD display is also fine.

To add to the mystery: when you set the front dial to manual focus, the aperture dance stops and remains on f/2.8. So what the has aperture open/close to do with manual vs single AF ??

Just for your info: i have no issues with the sound and everything works just fine, my technical background just want to know why this happens as this is clearly a (Fuji) mirrorless only thing.