What is the name of this video defect?

Solution 1:

It's a rendering issue, not an issue of your video file. Use a different video player or update your codec library and your video player. This is a screenshot on my system:

MPC-HC is a discontinued player, although on my system it renders fine. But if you are having issues with it, use VLC or something else.

To answer your question, there's no name for such defect, as it's not a video defect. It's impossible to say where on your system the problem arises.

EDIT: In light of dirkt 's comment, here's a possible explanation:

Motion compensation block boundary artifacts

Block boundary discontinuities can occur at edges of motion compensation prediction blocks. In motion compensated video compression, the current picture is predicted by shifting blocks (macroblocks, partitions, or prediction units) of pixels from previously decoded frames. If two neighboring blocks use different motion vectors, there will be a discontinuity at the edge between the blocks.

Source: Wikiwand.com

So, it can be that your player can't handle the discontinuity between the blocks in a sensible way.