Hooking the Gradle NDK/CPP Sync

Morning, I'm having problems with Gradle returning "C/C++ Configuration Problem" in the Event Log.

I have a few custom flags/defines that should only be passed with a certain build variant, but they're showing as "unknown argument" errors when I sync the project. Log will say:

Gradle sync started

Gradle sync finished in 30 s 84 ms

C/C++ Configuration Problem

MyTestApp | debug | x86

clang: error: unknown argument: '-fcustom-arg'

I'm trying to follow through the sync behaviour, but the CPP/CXX seems to end in a black hole. AFAIK I'm looking for the event/function that ultimately calls returns a GRADLE_CPP_SYNC_COMPLETED event.

openapi.externalSystem.util seems to just handle the java side of the Gradle sync, but something in-between ProjectSetUpTask::syncSucceeded logging GRADLE_SYNC_ENDED, and the sync completing, that I can't find to investigate this more