Earlier Xiaomi had pushed Android 8.1 OTA update along with June security patch which had few bugs which wiped entire SMS history after reboot due to this Xiaomi had stopped the update.
Now Xiaomi has worked on the OTA update and resumed the Android 8.1 update to Xiaomi Mi A1 starting from today. The update also brings latest July month Android Security update and OTA update weighs around 467MB.
But if you had earlier updated the device with Android 8.1 update that wiped SMS data then you need to download a huge 1.15GB file as it enhances and fixes the bug. Anyway, the rollout has already begun and might soon be hitting your device so be patient.