This happened to me to the 1st day I updated. I restarted the phone. The wall paper was back and haven't had an issue since. I was/am using a 3rd party wall paper.
Does it persist if you go in and change the wallpaper? Doesn't matter which one -- just change it, and then change it back to your preferred one, if you'd like.
Just happened to me. Using a fingerprint to unlock my phone (Smart lock has not worked right since the update) there was no wallpaper just icons. It's the same personal wallpaper I've had on my phone since Nov 2018 rebooted phone and it came back.