Hi megan_baker! This behavior is something that we evaluated and changed in v3.0. The current behavior continues to display the last known value in the display-only until the last known value is outside of the current window. For the window where the start time is before and the end time is after the issue, you can hover over the chart where the value should no longer be displayed and see that those values will stop showing up in the hover legend. As you already alluded to, the non-reporting values will stop reporting once the last known reported value is outside of the chart window.
As a short-term workaround until 3.0 is available, you should be able utilize the if() function in order to get what you need.
if(ts(my.metric, not key=value),ts(my.metric))
If you replace my.metric with your particular metric name and key=value with the incorrect tag, then the only displayed data should be values without that specified key=value. If the incorrect information is a particular point tag key, then you should use not incorrectKey="*". If the error is tied to a specific point tag value, then you should use not key="incorrectValue".