Google Calendar Sync issue in Xiaomi Max 2 (Android N 7.1.1 NMF267), doesn't ask for "storage" access
Answering my own question. 

I'm now on Xiaomi MIUI Global 9.0 Stable 9.0.3.0 (NDDMIEI) with Android N 7.1.1 NMF26F.

I found a separate app called Calendar Storage. This app shows up under Settings / Apps / All It doesn't show up under Settings / System Apps or under Settings / Apps / Downloaded which is probably why I missed it earlier.

After doing "Clear data" and "Clear cache" inside this app, my Google Calendar app's sync seems to be working.