Xiaomi has confirmed its MIUI 13 update schedule for devices using Chinese ROMs. The closed beta has already started rolling out for select devices and the OEM hopes to start rolling out a stable build of MIUI 13 by the end of January. Mi MIX Fold owners will have to wait until the end of April for a closed beta update.
https://www.notebookcheck.net/Xiaomi-confirms-MIUI-13-update-roadmap-for-first-and-second-batch-devices-with-China-ROMs.589247.0.html