I load an mp4 and go to a certain position on a I-frame. The time shows hh:mm:ss.nnn, so I copy that time position into a MP4Box GUI to split the mp4.
The splitted files aren't splitted correctly at the I-frame.
I looked at the time at the beginning of the clip and I found it has a delay of 080ms (.nnn): subtracting that to the time position and putting this one in the splitter works well.
I also tried in Adobe Premiere CC and there is the same 2 frames of difference.
So, I think there is a bug in displaying time.
EDIT: I found +40ms loading Avi and +200 loading mkv.
So the problem is avidemux has a variable delay problem in displaying time.
Frame editing should be done with Avidemux 2.5.6. Avidemux 2.6 is not a frame based editor: it is time based, hence your problem. HD (avc,264, ...) only works with 2.6.
ps.: Explained from user point of view.
Anyway I updated avidemux to the latest build and used another mp4 splitter and I made more tests and I found the problem is very evident with mkvmerge:
if you insert the avidemux timecode shown at I-frames, the resulting mkv has lot of frames not wanted at the end of the first file, instead if you insert the timecode -80ms (the same amount that is at the beginning) the split is correct.
But if you use MP4Box it seems to split an mp4 ok even with the original timecode.
It remains the fact that the same frame in Premiere is timecode-2frames (=80ms).
As Avidemux correctly detects the offset at the beginning, it should be easy to correct the time shown and start from 0ms at the beginning.