Same here friends!!! Even I'm facing the same issue from last 7-8 days. I used oneplus 2 prior today  this phone with same sim card, but there was never a problem with oneplus.Sometimes all the signal bars on Mi A1 vanishes which is strange and again it appears back within next second and i want to know its only the case with me or any1 else is facing the same. I request Xiaomi developers to fix this problem.