I have the same issue - I have a new LG Thinq V40, though first noticed this in my old Motoroloa Droid Turbo, I stopped using Verizon+ on that phone beacuse it was a repeated issue, but I wrote it off to being an older phone. Today was the first time I've seen it on my new phone I've had for 5 days, and it is on an OLDer message (three days old) that I know was received. It was a photo, with no text included. I have since sent and received both photos and messages with no issues, but this "unsent" notification appeared several days AFTER the original message.