Why did my data usage stats disappear from settings?

[AC Question]

Last week my settings showed my total data usage as 7.88 gb. The billing cycle says July 29 - Aug 28, which is correct, so I am still in the same cycle ( today is Aug 26). Today, my total usage shows 4.88, and the graph which shows the rate of usage is starting at Aug 12, even though the cycle is still set to the right dates.. Why did the first half of my usage this month clear out? Any idea how to prevent this?