90 degrees is ok but 180 degrees does not do anything. Except for rotating 360 degrees....
Which version of Avidemux do you refer to? Do you mean the "Rotate" video filter or the "Rotate" feature in the MP4 muxer?
Upon a quick test of the both named above with my current Avidemux build, I could not find any irregularities with 180° rotation. Works as advertised for me.