I believe you are correct on all accounts. First, I just found out that one of the problem orders included only 1 line item so that eliminates the odd character show up between multiple items.
It's funny you mention "guest checkout" because I am working on a spreadsheet showing the correlation between all the problem orders. So far it looks like they all either checked out as guest OR they registered their user account at the same time they checked out. So now I just have to figure out if that means anything. I am going to run some test orders doing both those things. I'll report back if I solve this.