Hello!
It's been a while since I've last used Avidemux (last time on windows platform, now on mac) and somehow the clipping + saving is ... off? I use the a and b points but instead of saving 1.5 sec clip it saves 6 sec clip and I don't know what I'm doing wrong here cause the a and b doesn't seem to be working that way? So saved clip is basically 6 secs long when it should be 1.5 sec long.
For frame-exact editing, you need to re-encode the output. In copy mode, only cuts aligned at keyframes are possible.