Full wipe before installing and Wipe Dalvik/Cache before reboot for refresh purposes.You just did wipe cache and dalvik and flashed 8.7.12 right?
Or did you make a full wipe? and then you installed the ROM?
Well I don't know about Chinese closed Beta, but this one is running smooth and no bug until now after 12 hours use. Only theme can be open through settings but not through app "Not available in this region" is the message that it gives.Mi Note 3 Based on Android 8.1 like China Closed Beta ?
Lockscreen, Status Bar, Notification Bar
Optimization - Face unlocking speed (07-12)
Fix - The problem of flashing screen in the process of fingerprint unlocking (07-12)
Fix - Fixed background in PIP mode
Fix - Fixed themes details loading
Fix - Fixed Nature sound missing in sounds
Fix - Security stops working [Redmi Note 4]
Optimize - New Security Center UI
Fix - No notification on lock screen [Mi 6]
Optimize - Media volume can be changed only when media playing (same as in MIUI 9)
Fix -The video is stuck when taking 1080P video [Mi 5]
New - AI Portrait mode for single camera devices [Android 8.x] (07-06)
New - AI Portrait mode for the front camera [Android 8.x] (07-06)
Fix - Phone can unlock without password/fingerprint/pattern [Mi 6]