The admin documentation on Quick Insights mentions :
Limitation: The report configured in the Quick Insight does not get automatically or manually updated when the same report is modified in the Report Builder page. 'An Exception Occurred' error message appears when the modified report type Quick Insight is queried in Slack.
It seems like in practice, there is no error message though and the report is just out of sync with the updated one from report builder. It's pretty hard to remember to update the Quick Insights whenever you modify one of the related reports. It would be great if this limitation could be resolved.