Problems with outdated messages

When working on updating the outdated messages sometimes the button "This translation may need to be updated. Show differences" is not shown (it should be located just above the text area box).

Instead of showing the difference (like here) I have seen two other types of texts:

1. "This translation may need to be updated." (Example)
2. A message showing a problem, such as "Following parameter is not used: %number%" (note that this is still an outdated message). (Example, updated by FuzzyBot).

Does anyone have any idea why?

This is bugzilla:48187. It was WONTFIX'ed, because it should go away in some years anyway.