Can't sync with my Forerunner 645 [beta]

Having recently installed Garmin Explore to sync an Instinct, I realised it will let users pair also Forerunner 645s, with a [beta] tag next to it.

Now, I totally understand what "beta" means but… is it supposed to at least sync it?

Every time I manually click the double arrow button to trigger a sync it tells (don't know the english version of it) "sync not executed" in red.

Did anyone manage to sync Garmin Explore and a FR 645 or any other [beta] watch?