Well, I've now rendered 2 files since the last one that didn't encode at the full 192 Kbps, and they've both rendered properly (at the full 192). I'm in the process of rendering the final one of 16. If that one renders properly my score will be 14 - 2, the 14 being the proper bitrate and the two with the degraded bitrate. My files are 720p 25 fps 1700 Kbps downloaded from Israel, which is apparently a PAL country. It's a weird combination, but I'm trying to stay as close to the original parameters as possible. The only reason I'm encoding at 3000 Kbps is that's as low as PD will go for this size file. In spite of that, they look very good to my eye.
I've compared the same MP4 and M2TS on a 60" HDTV and still can't see any difference. Also can't see any difference on screen shots magnified 3X. However, like you, I CAN tell the difference between a 106 Kbps AAC sound track and a 192 Kbps AC-3 one, so I replaced that one with the M2TS. However, the 177 Kbps one was fine, so I kept it. So in the end, I only had to replace one clip, provided the one that's working now renders properly.
One more option, in future if a clip fails to render at 192, I'm going to try 256 and see what that does. Involves another couple of hours of rendering, but if it doesn't happen any more often than it has in this job that's not too bad. Hopefully it will be corrected, but if it happens too often I'll start using H.264 unless/until PD fixes it.
In any case, it looks to me like you've discovered a fault in this program, confirmed by me (although mine is intermittent while yours seems to be constant), and you may want to report it to Tech Support.
Dell Precision 7510 Laptop
Windows 7 Pro 64-bit | Intel(R) XEON(R) CPU E3-1505M v5 @2.80 GHz
RAM: 32 GB
Windows Experience Index 7.5