I think it’s because it ends up with slightly different framerates in the caps somehow: 15/1 vs. 15000/1001.
Not sure what the best fix is here.
You could try something like ... ! concat name=c ! videorate skip-to-first=true ! image/jpeg,framerate=1/15 ! qtmux ! ...
That might lead to the frames from the second input file shifted slightly though to maintain equidistance on the output side.
Alternatively you could also put the videorate with the 15fps capsfilter after each jpegparse. This might also retimestamp things slightly or drop a frame every now and then, or maybe not. I’m not sure if this framerate difference is just heuristics in the demuxer or if there’s an actual difference. My guess would be it’s most likely the demuxer.