Might be the same issue as T158114

T65078 might also be the same issue.

Quite possibly the most annoying issue I've encountered.

I would appreciate having a single-sidebar option.
I'm not sure if the skin developers will add that option, so here is temporary workaround to add to your wiki's Timeless.css.

@media (min-width: 1340px) {
  #mw-content-block {
    display: block;
  }
  #mw-content,
  #content-bottom-stuff {
  margin-left: 14em;
  }
  #mw-content-wrapper {
    float: right;
    margin-left: -14em;
    width: 100%;
  }
  #mw-related-navigation {
    width: 14em;
    padding: 0 1em 0 0;
  }
}

Also had this issue. As a temporary workaround, go to the MediaWiki:Timeless.css page on your wiki and add:

table {
  border-collapse: separate;
}

To reproduce this issue, simply try creating a table with rounded borders on a wiki: