This is very likely related to the rollback gadget. When a user has made multiple edits and I view a diff of their most recent edit, I also see a link to bring up a diff of all the edits performed by the user in question against the last edit of the previous editor. This link is titled "← next-to-last editor". Until a few weeks ago, it was working fine. Now however, it appears to compare the edit with the current edit (usually itself) rather than the last edit of the next-to-last editor.
I also have Twinkle installed and this functionality is similar to its "Since" option. This option still works fine.
Steps to reproduce:
1. On [[ https://en.wikipedia.org/w/index.php?title=Vikramaditya&diff=796134067&oldid=796133126 | this diff page ]], you can see a diff by Miniapolis. This is the difference between 1 revision. The user has made a series of revisions. I have an option, presumably created by the rollback gadget, to view a diff of not just 1 edit, but all the edits performed by this user (in this series of edits). This option is provided by a link entitled, "← next-to-last editor".
2. The "← next-to-last editor" would/should normally link to [[ https://en.wikipedia.org/w/index.php?title=Vikramaditya&type=revision&diff=796134067&oldid=796006544 | this diff ]], but instead links to [[ https://en.wikipedia.org/w/index.php?title=Vikramaditya&oldid=796179178&diff=cur | this one ]] which is quite useless.
While I was pretty sure that Twinkle handles this correctly, it appears that the "Since" function is not much better. Perhaps the issue is that these diffs are not comparing the "latest" edit. In any case, these labels are confusing.