Entering text like r43420 will create a hyperlink to Fisheye. So will r43357, but in the latter case the link points to a page saying only "No changesets with that ID found. ".

The problem is that the link is always points to the Fisheye repository associated with the current project, regardless of whether the commit was made within the scope of that project. r43420 was within the /JST subdirectory and so the linked-to URL https://studio.atlassian.com/source/changelog/JST/?cs=43420 is correct. r43357 was made within the /PLUG subdirectory and so the URL https://studio.atlassian.com/source/changelog/JST/?cs=43357 is incorrect.