There seem to be two scenarios where the editors go haywire:

1. switch a project to the iOS toolchain, quite & restart creator, and try editing a C++ file. Highlighting and code-completion are completely missing

2. open a file with the native (x86_64) toolchain selected, and then switch toolchains to iOS. The code-completion will still act as if the native toolchain is active (e.g. Q_PROCESSOR_ completion will only show intel cpus).

Of course, in both cases the arm64 toolchain is used, not the simulator one.

What's surprising is that in the second case, the code-model inspector shows references to paths in the MacOS platform, not the iOS platform inside the Xcode bundle.