The Max Size (MB) limit isn't strictly enforced. Storage size is checked only when Query Store writes data to disk. This interval is set by the Data Flush Interval (Minutes) option. If Query Store has breached the maximum size limit between storage size checks, it transitions to read-only mode.
Reference:
https://docs.microsoft.com/en-us/sql/relational-databases/performance/best-practice-with-the-query-store