For me to investigate the problem, I entered back the SMS center number today from 10h to 17h... and my phone sent 10 SMS messages:
at 10:07, 10:10, 10:11, 10:36, 10:37, 10:53, 10:57, 14:12, 14:13, and 16:56 .
Now I disabled the Xiaomi sync and I'll see if the phone would stop sending SMS...