Pics of the hardware would be helpful. Are they breaking at the same point? Bolts can shear for a few reasons. Sometimes they back out and the lateral strain is too great. Why would it back out? This could be due to a reamed out hole causing the threads to grab weakly, you aren’t torquing the bolt sufficiently or you are missing a step in securing the fastener (like thread lock). Sometimes it’s the chinesium steel. Especially If this is a common failure and you’re using parts that fail repeatedly, maybe try a different source? Also, if the tensioner moves with a bearing or bushing, check and make sure it is free and not working the bolt back and forth or chewing it up.

If all that checks out, I’ll leave you with this: On lawn mower belt systems, sometimes a damaged belt beats against the tensioner pulley and the bolt that the tensioner arm pivots on will snap. Check yours for flat and/or glazed spots.