Yes I know this question is a year old, but there were no answers.
That is motion artifacting, and is caused by the video encoder. Since you are already at Q=100, it is possible the only thing you can do is use a different program to encode the video, such as X264.
It is not caused by the keyframe interval, since it is too short and the quality is too high. It is caused by the more subtle encoding settings such as lookahead frame for rate control, reference frame count, and most likely, by the motion estimation settings, such as the algorithm, range, and subpixel refinement, and possibly by not allowing the chroma channel to be used by the algorithm.
You can still use premiere for editing, but will need to export to an uncompressed format or be able to choose a different encoder for export (no idea if that is possible). There may be additional advanced settings in the default encoder that may allow you to use better motion estimation settings and fix the problem.