Avidemux - editing overlap problem

[hibbs]

Using avidemux 2.6.1.9.170329 64bit under win7. Two mp4 files have been flipped horizontally with their bit rate reduced by avidemux each with the same settings. They appear to have identical formats according to mediainfo. The first part is opened with avidemux, the second part appended, then the overlap deleted by setting B to an I-FRM in the appended part and A to the same scene change in the first part. Video and audio are both set to copy, output set to MP4 Muxer. The resulting saved file is damaged, the first portion is fine and plays to the cut point, thereafter the video is just blank grey. Audio is fine.
What am I missing or doing incorrectly?
Any help will be appreciated.

[jagabo]

What happens if you leave the overlap (append only)?

[sfu10]

Have you tried saving it to an MP4v2 format container after appending and cutting?

[hibbs]

Thanks for the replies.
The appended and cut video was saved in a MP4v2 container, appended and not cut saved to an MP4 container, both with no change. The original second part was reprocessed again by horizontally​ flipping and reducing the bit rate from 1500 to 1000 Kb/s. Still no change. When I get time I will try appending the original files without and with the overlap cut and report back.
Perhaps someone has a suggestion for some other free software that will perform these functions.
Thanks.

[hibbs]

I changed the order of operations to append, cut overlap, set horizontal flip and reduce bit rate then save with mp4-mux. The resulting file is fine.