Where did you find that Azur CDN supports stale-while-revalidate directive? In the MS documentation it is said "Currently we do not support stale and re validate feature and there are no plans to support it yet. Please let me know if you have any further questions. Thank you!". See https://learn.microsoft.com/en-us/answers/questions/316956/azure-cdn-should-respect-origin-cache-control.html#answer-346428
Crisan Adrian