I'm not usually a forum type of person, I've never joined the forum of any product I've owned. That I am posting this is proof of just how annoying the bug  outlined below is, and how shocked I am that however much I searched I haven't found anyone else who might have come across this major bummer. As the phone was brand new, I waited, perhaps an upgrade to MIUI 11 will fix the issue, and I waited and waited. And then soaked the phone, got a used Samsung and forgot about it. The Mi 9SE phone survived after all, but it's my secondary phone and I barely take it out of home, let alone use the camera. But now the MIUI 12 upgrade forced itself on the phone, and today I had to make a sharp image of something so I used the Mi9SE's camera as the Samsung made a blur in ambient light. The image turned out crisp as expected, but not before the camera asked me what mode to set for long-pressing the exposure  button, when it hit me: the camera has been updated! My heart throbbing in my throat, I switched to PRO Mode, actually made more accessible than before, and then I took my first shot. And my heart sunk. I cannot for the life of me believe that in the course of one year, ONE YEAR, Xiaomi, for crying out loud, not a single user, not a single beta tester, not a single engineer, developer, designer, or the brand manager's babysitter, not one slightly photographically inclined person in the whole of Xiaomi Mi 9SE's user base discovered that LIVE VIEW/CAMERA SCREEN IMAGE BRIGHNTESS FAILS TO CHANGE WHEN SHUTTER SPEED SETTING IS MANUALY CHANGED IN PRO MODE, thereby rendering Manual Exposure in PRO Mode entirely useless, as unless one has an exposure table in one's had, like one had to before the invention of the light meter, a century ago, one has no way of knowing what the image will look like, as PRO Mode also lacks any kind of exposure indicator, like actual film cameras do, which have no live view screen showing sensor output, that is, what the image will look like. It's 2020, for crying out loud!!! Yet one has to keep guessing and take several shots until one figures out the correct exposure. Basically you're shooting blind.  Xiaomi pride themselves with the incredible quality of the cameras in their phones, which is more or less true, but this is such a grave error that can easily throw off any customer who wants to use their Xiaomi phone for serious photography.
Below are six images as illiustration, the same scenario shot at three different shutter speeds, first a screenshot of the camera screen with the current settings visible, and then the resulting photo. I seriously hope Xiaomi will resolve this issue soon, because to me personally this makes the Mi 9SE a useless brick.

PRO Mode, Manual Exposure, ISO 3200, 1/8

PRO Mode, Manual Exposure, ISO 3200, 1/60

PRO Mode, Manual Exposure, ISO 3200, 1/1000