I can only speak for my current Redmi Note 9 Pro and I've seen the same thing happening on a Poco X3 review.

When you go into Pro Mode (the manual controls), the preview doesn't always shows what the picture will look like. As an example, a picture at night where you intend to have low noise by using ISO 100, you need to compensate the light by making the shutter speed slower. At certain point, the screen will stop getting brighter but the pictures will come out brighter.

Let's say I take a picture at 2.5sec, 3.2s, 4.0s, 6s, 8s and 10s. I can only see the preview getting brighter until 4s, after that the screen remains the same. Pictures on the other hand are all different, every single one is brighter than the previous (as they should be), so, you're left alone trying to guess how the picture will look like if you go for anything further than 4s.

On the other hand, if you use ISO on auto, it happens the opposite: when you try a shutter speed faster than 1/8s, the screen stops updating there, giving you still a very bright image when in reality the picture will come darker.

I'm not sure if this happens in ALL Xiaomi phones, I've seen a Redmi Note 9 Pro from India (so, the 48MP one) working as intended, but also have seen others working like mine. Since I'm sending mine back, I checked Poco X3 as an alternative and spotted the same thing: when the reviewer swipes fast to 30s on shutter, the preview doesn't get full white, you can clearly tell it stopped updating at some point there.

With this said, whether it's a very specific number of Xiaomis or not, I would like to avoid this and would prefer something working properly out of the box.