Hi Magical Unicorn, I'm using your modified Mi Fit with my Amazfit BIP and it has been working fine but this new version starts updating the watch that had custom firmware to an older untranslated version than the custom one I use. After checking the logs I found Xiaomi is pushing firmware upgrades ONLINE through a new tool within the APK called "HMFwOnlineUpgrade.java", you can look for it within:
\smali_classes4\com\xiaomi\hm\health\device\firmware\g$1.smali
\smali_classes4\com\xiaomi\hm\health\device\firmware\g$a.smali
\smali_classes4\com\xiaomi\hm\health\device\firmware\g.smali
These are very bad news as it will force online upgrades for all devices soon and you can't prevent it or even trick it by faking the firmware version on the .fw file or the firmwares.json.
For now I tested the new Mi Fit with Amazfit BIP, Mi Band 3, Mi Band 3 NFC, and this auto upgrade is only happening on the Amazfit BIP... for now.
Can you investigate and come back to me with some info or planning you may have to work it around?
Thanks ever so much for your amazing project!