A Google search turns up a surprising number of hits for "Subaru broken tensioner bolt".
Here are just two, the first one including a metallurgical failure analysis
Regarding the analysis:
"….Here's his assessment of the fracture. I couldn't upload the .pdf file because it exceeds the max. file size, so I'm just copy-and-pasting the text.
"You have a clear indication (red circle) of high-cycle fatigue at the circle. That was the start of the
break. The bolt was bending at that point, and did so for a fairly short time, until 3/4's
of its usable surface was compromised. It then did a final fracture. This was not a torsional
break, but rather a simple bend (like both ends of a see-saw being pushed down).
I see good case hardening all the way around the bolt (That is the silver "skin" on the edge of the
break). It started in the base of the threads, where it would fail if put in overload.
I see some scarring on the thread faces leading to this failure start region. It might have come
from the damage after it broke, but it lends credence to something being put into the threads
(filings in the surface) or it was not tightenend in all the way and this was the first point it stuck
This looks like an overload caused by a pressure point or some repeating out-of-balance.
It broke pretty quickly too, so whatever the event was it was traumatic and repeating.
The only thing I don't know is if the material was adequate in the first place (used weaker
metal). That would take some other analysis tools.
But my first indication is the bolt was not the issue."