It's not just the last column set that may need additional fragmentainer groups.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.h

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 78 matching lines @@
     LayoutUnit nextLogicalTopForUnbreakableContent(LayoutUnit flowThreadOffset, LayoutUnit contentLogicalHeight) const;
 
     LayoutFlowThread* flowThread() const { return m_flowThread; }
 
     LayoutBlockFlow* multiColumnBlockFlow() const { return toLayoutBlockFlow(parent()); }
     LayoutMultiColumnFlowThread* multiColumnFlowThread() const { return toLayoutMultiColumnFlowThread(flowThread()); }
 
     LayoutMultiColumnSet* nextSiblingMultiColumnSet() const;
     LayoutMultiColumnSet* previousSiblingMultiColumnSet() const;
 
+    // Return true if we have a fragmentainer group that can hold a column at the specified flow thread block offset.
+    bool hasFragmentainerGroupForColumnAt(LayoutUnit offsetInFlowThread) const;
+
     MultiColumnFragmentainerGroup& appendNewFragmentainerGroup();
 
     LayoutUnit logicalTopInFlowThread() const;
     LayoutUnit logicalBottomInFlowThread() const;
     LayoutUnit logicalHeightInFlowThread() const { return logicalBottomInFlowThread() - logicalTopInFlowThread(); }
     LayoutRect flowThreadPortionRect() const;
     LayoutRect flowThreadPortionOverflowRect() const;
     LayoutRect overflowRectForFlowThreadPortion(const LayoutRect& flowThreadPortionRect, bool isFirstPortion, bool isLastPortion) const;
 
     // The used CSS value of column-count, i.e. how many columns there are room for without overflowing.
@@ skipping 81 matching lines @@
 
     bool m_initialHeightCalculated;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnSet_h