To clafiry, it depends which codex mob you have selected when you close the codex. If you have clicked on a mob that is not tracked and then open the codex dialog via the open codex command the text is "track", if you open the codex dialog via a tracked codex mission or did have a tracked mission selected when you close it it will display "untrack" as text when you open it.
As the text is not changed according to the mission in the codex dialog you select it stays the same until you close it and the text on reopen is as I described above.