sudoku : when only one trivalue remains, there is not necessarily a BUG+1

The object of this post had been to prove that when there is a single trivalue, then one of its candidates ocurs exactly three times in its 3 units and that this is the true candidate.
However here is a counter example from link : where G6 is a sole trivalue without any of its candidates leading to BUG.

5 4 67 | 23 9 26 | 8 37 1
3 9 2 | 14 17 8 | 67 5 46
78 1 68 | 35 67 45 | 39 2 49
-------------+-------------+------------
78 3 4 | 6 18 17 | 5 9 2
6 2 78 | 59 4 59 | 1 78 3
1 5 9 | 27 28 3 | 67 4 68
-------------+-------------+------------
29 8 5 | 49 26 146 | 39 13 7
29 7 1 | 8 3 29 | 4 6 5
4 6 3 | 17 5 79 | 2 18 89