I am experiencing the same problem, on iOS15 beta 5.
I am able to show the dialog when presented directly in for example viewDidAppear, but when showing in succession to other permission requests, and also when presenting after another functions block callback is returned, it doesn't show.
When stepping through the methods in XCode I am able to see that requestTrackingAuthorization is executed and also returns a result, but the dialog doesn't show and the trackingStatus remains and NotDetermined.
Would love to see this fixed in the next beta.