If its flopping back, something is stripped or broken. I'd start by looking a broken spot welds or stripped teeth or gears.
After it flops back, will it power back up, or do you have to pull it back? They didn't make the stuff under the seat any more heavy duty then they figured they needed to, to get past warranty.

If a missing spring was the cause of your issue, I doubt replacing the spring is going to fix the problem anymore. What ever has been sliding past the spring latch is probably missing teeth by now. Gene