When driving or just sitting still I loose connectivity with hands-free, it usually beeps, displays "no phone connected". It last 3-10 seconds. It will reconnect. But this is a safety issue as well as annoying. Does this with PRIV as well as my husband's Samsung.
Bluetooth, in a perfect world.......
Android is known to have issues from time to time.... early Marshmallow was terrible, but things got better. As you are having issues with two Android Brands.... I'd suggest you check Honda/Nissan sites and see if they have recommendations.
I would suggest
1) Check to see if there are any software updates for the car(s).
2) Deleting the PAIRING of both the car(s) and phone(s), and recreate them.