I am having this same exact problem with my 2016 C-Max. For me it just started about a week ago, not long after I updated to the latest version of SYNC 3 (19205). In the 16 months I've owned the car before that, it never happened. I've been using the same phone the whole time (Android currently on 8.1) and with Verizon most of that time. I see this is an old problem for SYNC that happens for both Apple and Android but apparently there is no fix for it yet? I've just cleared the Bluetooth connection on both the car and the phone and at least temporarily that has stopped it from happening in my case but it looks like Ford needs to work on a patch for this.