so what EXACTLY did you do when fitting the belt tensioner and stepper dampener.

Clearly there is a cause and effect here and obviously something you did caused the problem.

So - in mind bendingly intricate detail - what did you actually do ?
Was anything disconnected or any wires rerouted at any point ?

Have you twisted the belt ?
You would probably have needed to take it off the idler cog to fit a belt tensioner, and all it would need is a simple twist below the printbed to reverse the direction of travel.
That's probably my current favourite theory.

Did you move or otherwise fiddle with th endstop.

have you changed the slicer profile in any way ?

Something you did caused the problem. I can think of a few ways this might happen (the top one being the twisted belt) - but I'd rather you detailed what you did and we'll see if any of that fits any of my current theories :-)