Sudoku Puzzle no 216 (Tough)

My C program created this puzzle and I keyed it into Andrew Stuart's grader and solver. It gave the puzzle a grade of Tough. Then I went through the solution with the Take Step button and I saw that the puzzle used a solving method I had never seen before. Andrew Stuart calls it a BUG or Bi-Value Universal Grave. Click on the Solve Online link below to see it in action. Two other online solvers said that it could not be solved by logic. I posted it on my Sudoku Puzzles Facebook group and a couple of the members replied saying that they had finished it. Try it yourself and see what you think:

Solve online

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