I did this same itinerary last week on Jl though in Business. I was also surprised to get off the plane from Dfw at Nrt and my Bangkok flight was only 2 gates away. No additional security screening. The JL Agent in the Flagship Lounge is very helpful and goes over you entire reservation with you making sure everything is ok.
Only problem with going on to Bangkok from Nrt is there is no PE on the Nrt-Bkk leg. I do this trip about 4 times a year. Prefer Cathay as they have PE all the way. Sometimes JL has a fare you can’t beat. I stopped doing the AA flights to NRT and HKG a couple of years ago. As soon as they get their act together again, I’ll come back. Hoping return from Bkk next month goes as smooth as flight over.