I have 28.6 and my app won't connect to the car as much as it used to be able to, but I have definitely been having connectivity issues even when I'm at home. It should stay connected to WIFI while in sleep mode, but for some reason it switches to LTE and then it loses connectivity (line thru bars). Only way for it to regain WIFI is for me to go into the car to wake it up from sleep mode and then it'll switch from non-connected LTE to WIFI and then my app will be able to connect to it.
And yes before anyone suggests rebooting the car, I have done that and still have the connectivity issues.
I didn't have any connectivity issues with 28.5 so 28.6 hasn't been a good update to me
Hoping next update fixes this!