Kit settings for compiler and sysroot have no effect when compiling in macOS

I tried to set up a kit in Qt Creator that would use a custom build of LLVM / clang, as well as a custom sysroot. Naturally I set up entries for the compilers in Qt Creator's kit preferences, and then created a new kit that used that compiler, and had a custom value for Sysroot.

However, when I built using that kit, it still used the compiler from the current version of Xcode (as returned by xcrun --show-sdk-path), and passed "-isysroot=Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform etc etc" to the compiler, rather than my custom sysroot setting.

These settings either should have the expected effect, or be disabled or removed in macOS if they're not supported. Preferably they'd be supported!