Am I missing something, or are you just concatenating the MPEG-TS segments and slapping an .mp4 file extension on the end? This is not an MP4 file, and won't play in anything that expects .mp4 files to be MP4 files. (It'll still work in a surprising number of places because most players ignore the file extension and sniff the content anyway)

To make this work properly, you need to mux it into an mp4 container. e.g.

    ffmpeg -i video.ts -c copy video.mp4

(there are, hopefully, libraries for doing this that don't involve bundling the whole of ffmpeg)