Last modified: 2011-07-18 07:51:47 UTC
I'm assuming this is caused by r23483.
diff=next will be relative to the given oldid, and since behavior with oldids has been fixed to work correctly that overrides your title parameter.
You'd need to provide a revision id from the other page in the diff parameter in order to override it more specifically.
Well, I can't do that within wikicode - nor without another http-hit - which was the point of trying to do it this way.
Closing invalid, it is not a bug that if a single revid is given, it is assumed that is the page that is wanted. Diffs between different pages is hardly supported anyway, and only if the two revisions are explicitly given.
We have Special:ComparePages now anyways to perform exactly this function.