I think it’s because it’s gonna be the first issue under the new EIC, right? With the new redesignOn newsstand.co.uk, most UK mags (from Vogue to Vanity Fair) are listed as coming out on the 1st or 2nd August, so it shouldn't be too long before we see those September covers.
UK Elle seems to be the only one lagging behind, with a release date of 8th August, which is the week after next.
I haven't seen the cover yet, but Jordyn Woods is on Cosmopolitan's September 2019 issue. Not sure if it's the US or UK edition.