In the following example we shouldn't see any red:
  <div style="display: grid; grid-template-columns: repeat(4, 1fr); width: 100px; border: solid; background: red;">
    <div style="background: lime; grid-column: span 2;">This_is_a_very_long_line!</div>
  </div>

The thing is that the item minimum size should prevent it to shrinking to the 50px width of its grid area.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1500137 for more information.