I see big brighter spot on the bottom of the screen

Hi,

Has the phone been dropped or gotten wet at all?

Post some images of the problem back here so that it can be seen. Here’s how to do this on Ifixit 기존 질문에 이미지 추가하기.

Try starting the phone in safe mode and check if the problem is still there.

Try wiping the cache partition and check if the problem is still there.

If still no good it may be a faulty display assembly flex cable connection on the systemboard or a faulty display assembly.

As a DIY repair all you can do is to open the phone and first check that the display flex cable connection is secure and if so, then to replace the display assembly and check if it now works OK. Here’s the Ifixit Xiaomi Mi Note Screen Replacement guide that may help. If doing this yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.