Out of curiosity I just tested it with kernel 4.18.14 and could not reproduce
the issue there, either. This most likely means it was some other update in the
meantime. I recently updated libdrm from version 2.4.93 to 2.4.96 which might
also possibly be one of the changes that could have made the artifacts
disappear. Nonetheless, I am glad that they are, albeit somewhat magically,
gone.