Very interesting post bro! I also switched from the China ROM to xiaomi.eu but I did it some months ago. I always thought xiaomi.eu ROM had some bugs compared to stock china one but then I also thought it was me that didn't notice those bugs before. This is weird as they should be the same firmwares with more languages and some improved translations.
Do you agree that stock china ROM is more stable and also way smoother than this one?
To revert back to stock ROM you have to use miflash and flash it via fastboot. Be sure not to check the "Lock bootloader" option not you may run into troubles. You can download the stock ROM from this XDA thread:
https://forum.xda-developers.com/t/miui-updates-tracker-mi-11-pro-ultra-star-mars.4263135/
Anyway if i.was you I wouldn't revert to stock ROM since not getting notifications is a totally no go for me. Notifications are what make a smartphone "Smart", otherwise it's just a very good camera that can take photos.
Have a nice day