MIUI is sort of a homebrew OS broadly based on Android. Some things work, others don’t. I wouldn’t be surprised if Xiaomi and OPPO will eventually go the way of Huaweii and develop their own custom OS to lock users into their ecosystem. It’s a China thing.
And while I’m a big fan of Xiaomi products in general, I wouldn’t use any of their phones without flashing the EU ROM first to ensure compatibility with free market tech. If you’re willing to give that a try, the folks at xiaomi.eu are working hard to maintain compatibility.