Thank you for your post. I'm sorry to hear about this audio/video synchronization issue you are experiencing. I'd be happy to help!
Let's try the following troubleshooting steps:
Perform a product reset:
Turn the soundbar off
Unplug the soundbar power cord from the power outlet
Wait 30 seconds
Reconnect the power cord
Wait 30 seconds for the system to reboot
Reset the Apple TV and Television
Check to see if your TV has any Audio/Video Sync settings that you can try adjusting.
Finally, I was able to find this thread within the Apple 4K TV Support Community. Some users found that disabling Dolby Atmos within the TV's settings helped. You want to be sure that both the TV and Apple TV's audio settings are set for Dolby Digital or DTS 5.1.