There have been many times, and several in the last few days ( https://github.com/apache/incubator-druid/issues/8291, https://github.com/apache/incubator-druid/issues/8263) when I wanted to label an issue as 'bug' but the semantics of the word "bug" don't really apply. For me, "bug" means "somebody made a programming mistake at some point". This doesn't apply to the issues linked above. I would say about them "the system, as it is currently implemented, fails (or may fail) under certain circumstances and shall be improved". PRs which would fix these issues shall be labelled 'Improvement'.
I think renaming 'bug' into 'defect' would be useful to broaden the applicability of the label.