As in title. When you edit milestone, version or component in repository (issue tracker) settings, sometimes you may end up with sudden remove of previously selected element.
At first, I thought, that this happens only on non-Latin characters (for example using correct "bēta", which requires UTF-8, in place of incorrect "beta"). But it turned out, that it could happen anytime. Simply edit some of your milestone, version or component, refresh page after successful edit and at some point you may end up with
default setting being cleared.
Note: You must refresh page or change configuration section to another and back to current to notice this issue. Even, if
default settings is removed after particular element edit, you won't notice this until next page refresh.
BTW: I'm marking this problem as "trivial", though I have a very bad feeling that fixing it won't be trivial for Bitbucket core dev team. Adding
delete link to auto-generated comments also sounded like trivial thing, but it turned out, that even after half of the year, you have failed to implement it. It seems, that there are no trivial things for you. Pity.