I have a theory about why I was seeing this error. I have 3 APs in the house, and I had set them all up with the same SSID, to allow devices to "roam" between APs as you walk around the house. With "inter-access point handover" enabled on the device (and the same SSID on all APs), it will switch to the AP with the strongest signal as you move, rather than waiting for the connection to drop completely before switching. The car was connecting to one AP as I drove in, but apparently not the one that's strongest in the garage, and it appears the car doesn't support inter-AP handover, so it would just hang on to the weak connection, and then it would fail sometimes in the garage when I got back in.
So I changed the APs to all have different SSIDs, and set the car to only connect to the one that's strong in the garage. So far so good, but it has only been a day.
This may not be the cause for everyone, but it supports the theory that the message comes up when the car is associated with the AP on WiFi but either not getting an IP address or experiencing high packet loss due to a weak signal.