Simple math. Example: If there are 100,000,000 first generation spring migrants in the wild, very roughly around 5% (5,000,000) of them will be in Minnesota in May. If butterfly breeders ship around 300,000 adults and caterpillars per year for release, mostly in April-October, that’s only a maximum of 50,000 per month. Of that 50,000, only around 5% (2,500) would be shipped to Minnesota per month.
5,000,000 divided by 2,500 = 2,000. So wild monarchs in Minnesota would outnumber the captive raised ones by a factor of very roughly 2,000 to one. Thus the chances that a Journey North observer in Minnesota (or the Canadian provinces north of Minnesota) would encounter a captive raised monarch in the month of May would be in the neighborhood of 2,000 to one (an extremely remote chance).