Hi my site was recently de-indexed on bing.
I have same issue on my site . You need to add meta tag on the page where such problem your facing . I had same issue on my usps tracking site. Please refer following screen shots of error i found.
i contacted bing webmaster support.. but they suggested only to refer their guidelines . No more reason they share.
I just putted meta tag between <head> </head> tags.
<link rel="canonical" href="https:// yourdomainname.com/pageurl/" />