I would be really surprised if there isn’t a way for the crew to lower the seat into flat mode and raise it again later by manual means; indeed there might even be a requirement for that, since what would happen if a passenger had been in full recline and then the seat wouldn’t power back up to full upright? You cannot have a passenger sit/lie there for landing, and if no other seats are available that’s not acceptable. If not, that is a design flaw at the very least. I was an FA for UA for a number of years and our Biz seats could be manually lowered and raised by means of several hidden release levers beneath the seat. We weren’t taught about it per se, but the pursers knew and that’s who I learned it from, and I had to use it several times. Either way it’s DL’s responsibility to address this in flight.