Thanks for the suggestion,
My procedure is:
–shoot video with panasonic camera (various models) at 1080/60p
–process color, contrast, etc., remove native audio - using Avidemux. (Default is Constant rate factor and I don’t change it.)
–save as mp4 at 1080/23.976p (some clips slow motion, others resampled at full speed)
–combine various clips and still images using Shotcut
–add audio track
–export as mp4 at 1080/23.976
So if “constant” is equivalent to “fixed” then that is not the issue.
Avidemux encoding options are Constant Rate, Constant Bitrate, Constant Quantiser, Average Bitrate.
Would any of these solve the problem?