I installed the latest "v2 font" firmware, having a custom v1 font installed. It didn't complain at all and worked as a charm! I installed the v2 font and no complaints either. Everything was fine!
Then, out of curiosity, I changed the language (from notify & fitness) to Korean, to check the new v2 font. After doing that, my band asked to connect to Mi Fit to update. I couldn't change the language back to the previous one or any other language. It was fixed only after installing a v1 font.
Conclusion: the "beta" firmware is still at "beta" stage!