To reproduce:

• Fork any repository to your personal area
• Navigate to compare between two tags within this forked repo.
• Save the URL
• Bring up the same URL in another browser or tab. For the target, it uses the original repository, instead of the forked one. You can see that in the URL, the target repo is not saved anywhere.

Here's the URL that I got: https://stash..../users/maloneyc/repos/jats-tag-libraries/compare/diff?targetBranch=refs%2Ftags%2F1.1d3-20150316&sourceBranch=refs%2Ftags%2F1.1d3.