I wasn't able to check in Sierra, because I've got only High Sierra and Mojave backup volumes. (I've got a Sierra installer, though, and if push comes to shove I'll run it.)
But since you said your issue happens in your husband's HS as well as your Sierra I booted into HS to see what would happen, and I had no problems on Amazon. (This was in Safari 12.0 to begin with and 13.0 after I finished updating.)
BUT, I noticed something in HS (Catalina too, so it's not version specific) that never caught my eye before, namely, that when I hover over the review stars on a product list page a little empty white box flashes on my screen for a fraction of a second before the stars box comes up.
That, combined with your review issue on specific product pages, smells VERY MUCH to me like a content blocker issue, so I'll suggest that you try turning yours off and back on one at a time to see if you can nail a culprit. I hope you're running content blockers anyhow, because if you aren't, I'm out of ideas. (I'll note that I've run into review/content blocker conflicts, but not on Amazon.)
PS: To post a screenshot or other type of image...
