I have the same problem on my 1990 where all the main buttons work but the side seat release lever/button will tilt the seat up, but it only moves the seat about 50% of the time. There must be a short in that release button, as I can always move the seat up with the main control button. Same problem as you are experiencing. Seems common. Any ideas on where the failure point it located?
From my research, both 780s Gen 1 and 2 had memory glide features. There was a difference in the release button on the side, which changed in 1990. Volvo engineer Sven-Gunnar actually redesigned it on a napkin on a flight from Turin to Gothenburg around 1989. There were concerns for ease of use, as the newer button sticks out and is easier to hold. I can find out who much of the technical aspects are different. There were safety concerns with the Gen 1 that may have resulted in different technology in order to make sure the seat stops and backs up if someone is trying to crawl out and the seat could potentially crush the if it didn’t stop.