@RicRocha 

Both APs are running the latest firmware (by which I mean even beta firmware)?

Since these are IOT devices, it's likely they may need looser timing setting for their radio comms or may not support things like fast roaming that you may have inadvertently enabled.

An interesting test would be to disable the 670 temporarily and see if those D1 minis are stable when there is only 1 AP to connect to.  If yes, it's a roaming support issue, if not it's an AP setting.