Had this happen to me for the first time last night and at the first traffic light I restarted the truck and it didn’t help. I did a force restart in sync and as soon as it came back up it instantly connected again to CarPlay. Just hold down the audio power button and the skip forward button on the radio together for about 5 seconds and you’ll see the screen reboot.
Going from Sync3.4 to Sync4 I have noticed that Carplay launching is very inconsistent. With Sync3 19/20 vehicle starts both Carplay and Android Auto connect within 5-10 seconds after the infotainment system completed its boot. On rare occasions I would have to unlock my iPhone for Carplay to launch. It didn't matter if the phone was plugged in before or after power on.
With Sync4 it's much longer when it does connect, to the tune of up to a minute or more post boot and 2-3 of 5 startups requiring me to tell Sync4 to use Carplay manually fromt he phone menu. This is with both wired and wireless Carplay, but wired does seem a little more consistent. And I mean a little like in little...
So far I have completed a master reset on Sync4 as well as removed the truck from my iPhone three times and reset the link. So far none The only things I have found that do make Carplay launch a little more consistent is using wired over wireless as well as turning off WiFi app projection. Since doing that Carplay usually launches within 15-30 seconds of system boot. It also does seem to be a little better if the phone is plugged in prior to system boot than the sync in mid-boot or has system already booted.
Remember, SYNC & CarPlay are delicate flowers and need to be treated with patience, love, and affection. Never make them mad! For instance, if your phone is connected to CarPlay, do not get out and walk away from a running truck with the phone in your pocket. They get very upset!
Seriously though, it is a bit temperamental. When I have issues, I reset my phone. I almost always get a prompt on my phone afterward to 'finish connecting CarPlay'. I think the issues are on both ends (SYNC and Apple).