Honestly, I'm not sure where the issue lies. I have taken your image and tested it with STSM and I get this much more sensible result.
I would recommend you use this slope, STSM cannot deal with overhangs but I cannot guarantee that it will fix your issue. 
I assume you're already using the most recent version of the tool? If not, download it again.
45 days ago(+1)
It worked thanks I guess I can rotate the result to complete the loop! Thank you so much for this tool by the way saved me countless hours :D