The Weekly update 9.12.5 has been officially released for Mi MIX 2S (codename is polaris). Before updating, please make sure you have downloaded the exact ROM matching your Xiaomi phone/tablet. In Fastboot mode, you can check model code by typing fastboot getvar product in cmd/shell.
The firmware files posted here are all official Xiaomi MIUI or HyperOS ROM, xiaomiROM.com do not own and never modified those firmwares，you will always download the file from Xiaomi server.
You may interest in those contents: Mi MIX 2S ROM Topics Xiaomi Devices Index
Download Mi mix 2s (polaris) China MIUI 11 9.12.5 Recovery ROM with Android 10.0. Firmware size is 2.5 GB.