I had it happen on a 4th gen - hasn't happened yet on my 5th
For those worried about cargo - if you have your rambox bed divider, you can use that or a cargo net to keep everything secure, even if the tailgate came down
ETA: And, if you ever suspect your tailgate fell?
you go through uconnect, you can take a look at your rearview camera - if driving, you can have a passenger do this. It will give you a view for a brief moment. If you see the road behind you and cars behind you? It's up.
If you see asphalt? It's down.