I am a USA citizen wanting to use the 72 hours layover rule on my way back to USA

If I am coming from Bangkok and arriving to Beijing PEK by plane July 22 7:20am and leaving the airport to explore. Then leaving PEK July 23 at 5:15pm back to Los Angeles LAX, but I have a layover before LAX in Shanghai PVG July 23rd at 7:25pm (not exiting the airport). Am I still valid for the 72 hour visa? I see that I am under 72 hours, but does my layover in PVG mess things up?