According to the Chinese insider Digital Chat Station, Xiaomi is already testing MIUI 13. This is probably happening in China, where the updates are initially released.
Also, the insider is sure that the new firmware will be received by devices that have been released over the past two years. That is, for example, Mi 11, Mi 11 Pro, Mi 11 Ultra, Mi MIX Fold, Mi 10, Mi 10 Pro, Redmi K40 and others. This is not surprising, since usually Xiaomi provides three MIUI updates for flagships, as well as popular mid-range devices, and two MIUI updates for budget smartphones.
The Xiaomi Mi Mix 4 is rumored to debut with MIUI 13 on board. The official announcement is expected in the third or fourth quarter, meaning the latest version of MIUI should also be announced around the same time.