Probably you have already solved this problem, but there might still be people out there who will encounter it, therefore I reply here ☺
I had the same problem with a fresh installation of Premiere and found out that Premieres multicam sequence preset is a Arri Alexa. If you just create one new multicam sequence with for example DSLR 1080p it will default that in the future. And that preset worked for me filming multicam with 2 iphones and external mono audio.