Category of issue in mail subject does not change when category of an issue is changed

Emails of issues now have in their subject something like:
[bug] Issue title

Where the category of the issue (bug, task, support, feature) are put in square brackets.

However, when the category is changed in an issue, this change is not reflected in the email subject for future followups.

In case I'm not being clear, here's what happens:

                     d.o issue category             email category indicated in []
new issue created       bug                                bug
category changed       feature                           bug