Steps to reproduce the issue

In default core template of Joomla Blog view:
Around line 110 in com_content/views/category/tmpl/blog.php or in any override equivalent:

<?php if (($rowcount == $this->columns) or ($counter == $introcount)) : ?>
</div><!-- end row -->
<?php endif; ?>

This causes layout break when choosing more then 1 columns
All DIVs with class="span-etc" are wrapping others and others and others forever, like an inverse piramid

Removing the IF statement (remaining only the closing div < / div >) all works fine. :

Expected result

<div class="row
 <div class="span6">
   etc
  </div>
 <div class="span6">
   etc
  </div>
</div>
<div class="row>
 <div class="span6">
   etc
  </div>
 <div class="span6">
   etc
  </div>
</div>

Actual result

<div class="row>
 <div class="span6">
   etc
  </div>
 <div class="span6">
   etc
      <div class="span6">
       etc
     </div>
     <div class="span6">
       etc
     </div>
  </div>
</div>

System information (as much as possible)

Joomla 3.8.6
Default Protostar template with default category blog override
or
Default Protostar template without override (core blog.php view)

Additional comments

My modification is a bad solution, this is the fact. But it's a solution :)

My result is a 'div class="row"' inside another 'div class="row"'

But the columns are aligned....