I am happy to provide evidence tomorrow during day time if the light conditions are similar, if people are not bored of this thread already...
but I did provide an iso 1600 image that is significantly higher than 100, and I still got a very dark photo. It looks better in the iPhone photo I took of the Z6ii screen, the iPhone did a lot of corrections, in real life you could barely see a thing on the screen.
ISO numbers can seem deceptively large. ISO 1600 vs. 100? Sixteen times as big! But in terms of exposure it's only four stops. 20000! Huge, 200 times bigger! But it's 6-1/3 stops. I'm using overly-dramatic language here but don't mean to be insulting. But it's common to equate large scalars with large effects when in fact they may not indicate such a large effect in real life. (Another scale going the other way is earthquake magnitude: from 6-8 doesn't sound like much - it's only 1/3 more in terms of numbers, but the magnitude 8 quake is 100 times stronger than the magnitude 6 quake.)
It's a lazy Sunday morning so I did my own test shots. These scenes are in my kitchen, which is not lit by full-length south-facing windows. There is one 1x1.5 meter window on the south wall but it's shaded by a tall tree. I have a skylight fairly close to this scene. The scene is also lit by a bank of undercounter LED lights. Otherwise there is no other light in the room. I'm in the US Pacific Northwest, which is a similar latitude to London (I'm assuming you live there due to the Underground mug) so right now sunlight strength will be relatively close to there. My mug is very similar to yours in color ...
I shot in both manual and aperture-priority automatic. For each photo pair, first is the scene from behind the camera, taken with my phone and showing the settings on the monitor; second is the photo my camera shot. You can see that at the same ISO values you illustrated my scene is lit very closely to yours.
Setting the scene. Note the settings on the monitor.
Actual image taken with settings above: ISO 100, 1 second, f/7.1, aperture-priority automatic, matrix metering. It's a nicely lit scene that shows the white mug without blowing out its highlights. The mug was shadowed as illustrated.
Shot at your original image's settings. No idea here why the camera didn't show the exposure over/under indicator. It does in subsequent shots.
Severely underexposed at settings above: ISO 100, 1/60 second, f/7.1, manual exposure mode, matrix metering.
Raised ISO to 1600. Scene is still underexposed even at four stops higher gain.
Image from above settings: 1/60, f/7.1, ISO 1600. Still underexposed.
Manual exposure indicating correct exposure (exposure indicator at right indicating no under- or over-exposure). 1 second, f/7.1, ISO 100. The same exposure that aperture-priority auto-exposure indicated when the focus box is on the center of the mug.
Exposure from above. Mug is correctly exposed without blowing out highlights (not too much, anyway).
Just for grins, I then pointed the focus indicator at different areas of the scene and noted how the exposure over/under indicator changed. I didn't bother taking pictures with the camera.
Still the same manual exposure settings, but with focus box pointed at inside of coffeemaker. Note it is now showing a stop under-exposed. Matrix metering weights towards the area indicated, but still takes the whole scene into account.
Focus box aimed at tiles next to coffeemaker. Still shows 1 stop under-exposed.
I hope these images illustrate things in a way that helps you understand why your image was so underexposed while also giving you confidence that your camera's meter is not at fault.
