There are reports that the first MIUI 12 build for the Mi Mix 3 5G has removed 5G connectivity for the device. The EU variant is the only one to have received Xiaomi's latest too, albeit one still based on Android 9.0 Pie. All others remain on MIUI 10, over 18 months after being released.
https://www.notebookcheck.net/5G-connectivity-woes-plague-the-Xiaomi-Mi-Mix-3-5G-on-MIUI-12.494415.0.html