I previously posted this in suggestions, but now i think this is a bug.
At this moment (12.09.2019), XenForo in RSS feed pushes into guid just link to thread: This can be result as problems for identifying thread if author changes thread name, and if forum uses setting named as "Include content title in URLs" (/admin.php?options/groups/seo/#includeTitleInUrls). I...
<guid>should contain unique identifier for thread. This can be as canonical link without title, and just a thread identifier.