Which ffmpeg flag fixes MP4 missing frames in Premiere?
Solution 1:
Having tested a few scenarios, it's the presence of multiple B-frames that looks responsible. It's their storage out-of-presentation sequence and a new bug in Adobe's H264 bitstream parser which is causing this problem.
So, adding -x264opts bframes=1 (or 0) solves it. This is not an issue with FFmpeg, AFAICT, since other apps including NLEs like Vegas can read MP4s without this option correctly. Even older versions of Adobe CC do, as per your links.
Edit: -x264opts b_pyramid=0 looks to be the direct solution.