It appears that this a bug in Uncrustify itself; you can't necessarily work around it with a config change.
See:
https://github.com/bengardner/uncrustify/issues/68
(Personally, I'm a big fan of turning on "tab always indents", then selecting all, hitting and living with whatever Xcode produces. By having a standard set of formatting defaults for our team, anyone on the team can do this and not generate a bunch of spurious diffs when committing code.)