I've not had any problems with my Mi scale and it's quite accurate compared to another older one I have at home.
But it is interesting that yours failed just when Xiaomi released a new version 2 . Maybe they send a wifi command out to bugger up all the scales so that everyone goes and and buys version 2. But anyway, only $99 is a great price and just throw it away and get a new one!
https://buy.mi.com/hk/item/3192200018
They have regular calibration schedules to check the accuracy.
There may be some useful information here: https://www.youtube.com/watch?v=DeEOUoL3uVk
A simple test of the accuracy is to step on it *with* and *without* holding a 5kg sack of rice.Subtract the two readings.
Even if consistent within one session, it may drift between sessions due to temperature, humidity etc.
Calibration is not an issue here- the OP is talking about multi-kilo variations within a day. And the temperature drifts mentioned earlier in this thread were after putting the scales in a freezer. I strongly doubt that a handful of degrees, or a couple of tens of %RH change, will result in that amount of variation.
Depending which part of angle I step on. I get different results too. Typical Chinese quality.
2 kg bag of rice showing as 4 kg!