I had a porch with a similar looking transition leak on me once. The existing house didn't have a porch, and they wanted it to go out as far as possible. I think I went 12 feet. The house was a 6:12 pitch that was about ten years old, and I built the porch roof with a 1:12 pitch. After a heavy rain, there would be several leaks right at the transition where the flashing was that leaked into the porch area. I went over it inch by inch and couldn't find anything. I ran the hose over each panel right above the flashing and it never leaked. I spent hours out there over several days and could never get it to leak with the hose, but after a heavy rain, it leaked again.
Then I got the idea that maybe it wasn't anything that I had done, and maybe the existing metal roof was leaking. I put the hose at the peak of the roof and let it run. Sure enough, water started coming out at the flashing. What I realized was happening was the rubber gaskets on the existing roof where leaking and had always leaked. The builder put the metal over felt paper, over OSB decking. The water went past the gaskets in the screws, and then under the metal, on top of the felt paper. They had gutters there, so nobody ever knew it was doing anything unusual because the gutters caught all the water.
I replaced the screws in the roof and the problem was solved.
Eddie