Having looked again at the graph in the GAO report, and done some searching on the Net, I'm not sure that there is a strong link between declared F135 reliability and the cancellation of the F136. The F136 was effectively stopped in July 2011, and fully stopped in Dec 11. The GAO graph shows a jump in reliability in early 12, followed by a major fall in late 12.
Lag between production and in service use. The spike would have been with the last engines produced in the final push to get the F136 cancelled. The stats thereafter with engines manufactured after the cut off.....