combining smartphone video clips changes the aspect ratio

[schaloule]

I have taken a few short video clips with my smartphone and I want to combine them to a single video.
Playing the clips with VLC on my windows system looks the same as on the smartphone.
But when I load those files into avidemux the aspect ratio changes from portrait to landscape.
When I save the concatenated result and play it with VLC the aspect range is not shown correctly anymore.
I have configured avidemux to copy, and I wonder why the aspect ratio is not kept to the original.

How can I preserve the aspect ratio of the original video?

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[eumagga0x2a]

You probably got source videos with the same resolution and the same aspect ratio but with different angle of rotation (0/90/180/270 degrees). The latter property is not evaluated by Avidemux by design. It is just a hint for video players (for the part which is responsible for display). One can set it manually in the configuration of the MP4 muxer in Avidemux (no other container supports this metadata), but this makes sense only if all rotation metadata of all appended videos match.

In your case, the only solution is to re-encode source videos which need to be rotated with rotate / crop / zoom filters actually transforming the picture to match the remaining videos. To avoid increased quality losses from an additional generation of re-encoding, you should consider using a lossless codec like Ut Video or HuffYUV (or at least H.264 with the Mezzanine preset) on condition that you have a plenty of free disk space.

Merging in copy mode won't be possible (it requires all codec settings of all involved videos to match perfectly).

(I don't get the point of attaching mediainfo output for processed videos. Only the output of mediainfo for unmodified, original source videos might be helpful.)

By the way, you should use the unaccelerated "Qt" (labeled as "RGB") video display in Avidemux only when all other options crash. On Windows, the best video display performance-wise is "DXVA2" (which is simply DirectX). With the unaccelerated "Qt" display, all the colorspace conversion and scaling is done on the CPU, which is terribly inefficient in comparison to doing scaling on the GPU.