A developer who likes to dig deep into Xiaomi’s software to look for clues suggests that the company is ready with MIUI 13 stable release and the uncovered builds are for the vanilla Xiaomi 12.
— Kacper Skrzypek 🇵🇱 (@kacskrz) December 1, 2021
The latter is rumored to come this month so the time frames coincide well and Xiaomi will likely introduce its new skin with Xiaomi 12, which will come with MIUI 13 based on Android 12 out of the box.
That builds are for the Chinese version of the phone, but the global variant will likely follow these steps.
The same source also has a list of devices that should expect Android 12 update with MIUI 13 on top so go check those out.