When you disable a text format it is not possible to re-enable it. This seems counter to the behavior in other situations. For instance, in Views, it's possible to Disable a display and Enable it later.
And here's how Views works:
Beta phase evaluation
|Issue category||Bug because of unexpected behaviour. Upon disabling the text format it disappears from the form which makes it seem deleted, but it isn't actually deleted because you can't add a new text format with the same machine name.|
|Issue priority||Major because as cilefen states in #19:
|Prioritized changes||The main goal of this issue is two-fold, fixing a DrupalWTF bug and improving usability.|
|Disruption||Non-disruptive because only the filter format admin interface is changed. No API changes|