Maybe because I'm left-handed, I encountered this glitch.
With Filmic Pro, Avidemux will only read iPhone videos shot with the home button to the camera operator's right.
If you shoot with the phone's home button to the operator's left, Avidemux reads the file upside down. And I haven't found an easy fix. None of the Filmic Pro settings seem to affect this behaviour.
None of the other software I use has this problem (QuickTime, VLC, Davinci Resolve). They all run the file in the expected orientation.
I assume these files are MP4 / QuickTime files (AFAIK no other formats support rotation metadata). Avidemux doesn't evaluate rotation metadata and never rotates preview (implementing the former and passing it down the chain would be an enhancement, the latter is by design).
edit: The design is to display video always the way it is coded, without any transformations hinted by metadata (i.e. no anamorphic display = all pixel are displayed with 1:1 aspect ratio, no rotation, no mirroring). This design choice is related to handling of multiple appended source videos, where the way the videos are encoded is crucial for the outcome.
Of course, when saving output to MP4 / MOV, you can set the rotation hint in the configuration dialog of the corresponding muxer.