Coming out of Costco last week, I opened the rear liftgate on my 2021 Exp ST and loaded the groceries.  The liftgate opened fine, but would not close no matter what I tried.  I finally had to close the liftgate manually by pushing it down to close.  That has been that way until yesterday.
I spent every day since then researching and googling to find a solution.  I even purchased Ford’s Service manual, thinking it was a wiring or mechanical problem.
My wife just told me yesterday about a woman on YouTube having the same problem with her 2020 Ford Expedition.  She cleaned all the sensors around the liftgate and it started working again.  I said, no way! That’s nonsense, nothing is ever that easy!  
So, when my wife left my man-cave office, I took a fiber towel and a small bucket of water, ran outside and cleaned all the sensors around the liftgate. 
Honest to God truth, that S.O.B. liftgate started working again! I said, no F*#$@n^ way!  I tried 8 to 10 different times using all 4 ways of opening and closing the liftgate and it has worked every single time! Lol… Not telling my wife.  I won’t hear the end of that one.